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OSSSC Combined Recruitment Examination study materials
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OSSSC Combined Recruitment Examination practice questions

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Question 1General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

The Fundamental Duties of citizens were added to the Constitution of India on the recommendation of which committee?

  1. Balwant Rai Mehta Committee
  2. Sarkaria Commission
  3. Ashok Mehta Committee
  4. Swaran Singh Committee Correct answer.

Explanation

The Fundamental Duties were inserted by the 42nd Amendment Act, 1976, on the recommendation of the Swaran Singh Committee, which suggested a set of duties for citizens. They were placed in Article 51A under Part IVA of the Constitution.

Question 2General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which river forms the boundary between Odisha and West Bengal over a considerable stretch before draining into the Bay of Bengal?

  1. Subarnarekha Correct answer.
  2. Baitarani
  3. Budhabalanga
  4. Rushikulya

Explanation

The Subarnarekha rises in Jharkhand and flows along the Odisha-West Bengal border region before entering the Bay of Bengal. The other rivers listed lie entirely within Odisha's drainage.

Question 3General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A shopkeeper marks an article at 25% above cost price and then allows a discount of 20% on the marked price. What is his profit or loss percentage?

  1. 5% profit
  2. No profit no loss Correct answer.
  3. 5% loss
  4. 2.5% profit

Explanation

Let cost price be 100. Marked price becomes 125, and a 20% discount reduces it to 125 × 0.8 = 100, which equals the cost price. Hence there is neither profit nor loss.

Question 4General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

In a certain code, ODISHA is written as PEJ TIB. Using the same rule, how would CUTTACK be written?

  1. DVUUBCL
  2. DVUUBDL Correct answer.
  3. EVUUBCL
  4. DVTUBCL

Explanation

Each letter of the word is shifted one place forward in the alphabet: O→P, D→E, I→J, and so on. Applying the same rule to CUTTACK gives D, V, U, U, B, D, L.

Question 5General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which Article of the Constitution of India abolishes untouchability and forbids its practice in any form?

  1. Article 18
  2. Article 17 Correct answer.
  3. Article 15
  4. Article 16

Explanation

Article 17 abolishes untouchability and makes its practice in any form an offence punishable by law. It appears among the Right to Equality provisions of Part III.

Question 6General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

The Hirakud Dam, one of the longest dams in the world, is built across which river?

  1. Tel
  2. Brahmani
  3. Mahanadi Correct answer.
  4. IB

Explanation

Hirakud Dam stands on the Mahanadi near Sambalpur in Odisha and is a major multipurpose project for irrigation and power. The Brahmani, Tel and IB are separate river systems.

Question 7General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

The average age of 8 members of a family is 30 years. If the age of the youngest member is 2 years, what was the average age of the family at the time of the youngest member's birth?

  1. 30 years
  2. 28 years
  3. 32 years Correct answer.
  4. 34 years

Explanation

Total age now is 8 × 30 = 240 years. Two years ago the seven older members were each 2 years younger, giving a total of 240 − 2 − 14 = 224, and 224 ÷ 7 = 32 years.

Question 8General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following is the highest mountain peak of Odisha?

  1. Dhauli
  2. Barunei
  3. Deomali Correct answer.
  4. Mahendragiri

Explanation

Deomali, located in the Koraput district of the Eastern Ghats, is the highest peak in Odisha, rising to about 1,672 metres. Mahendragiri is a prominent peak but lower in elevation.

Question 9General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A man walks 5 km north, turns right and walks 3 km, then turns right again and walks 5 km. In which direction is he from his starting point?

  1. West
  2. North
  3. East Correct answer.
  4. South

Explanation

Starting north 5 km, then turning right (east) 3 km, then turning right (south) 5 km brings him back to the same latitude as the start but 3 km east. So he is east of the starting point.

Question 10General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Who was the first woman Chief Minister of Odisha?

  1. Basanti Devi
  2. Nandini Satpathy Correct answer.
  3. Sushma Swaraj
  4. Gamharia Devi

Explanation

Nandini Satpathy served as Chief Minister of Odisha in the early 1970s and is remembered as the state's first woman Chief Minister. She was also a writer and Rajya Sabha member.

Question 11General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following is the correct sequence of the layers of the atmosphere from the Earth's surface upward?

  1. Mesosphere, Troposphere, Stratosphere, Thermosphere
  2. Stratosphere, Troposphere, Thermosphere, Mesosphere
  3. Troposphere, Stratosphere, Mesosphere, Thermosphere Correct answer.
  4. Troposphere, Mesosphere, Stratosphere, Thermosphere

Explanation

The atmosphere is layered by temperature behaviour: troposphere nearest the ground, then stratosphere, mesosphere and thermosphere. Weather phenomena occur mostly in the troposphere while the ozone layer lies in the stratosphere.

Question 12General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

In a class of 60 students, 35 play cricket, 30 play football and 10 play both games. How many students play neither game?

  1. 10
  2. 20
  3. 15
  4. 5 Correct answer.

Explanation

By the inclusion-exclusion principle, students playing at least one game = 35 + 30 − 10 = 55. Therefore students playing neither game = 60 − 55 = 5.

Question 13General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following pairs correctly matches an Odisha dance form with its description?

  1. Odissi - classical dance known for Tribhangi posture Correct answer.
  2. Dalkhai - classical dance of Kerala
  3. Chhau - classical dance of Tamil Nadu
  4. Ghumura - folk dance of Rajasthan

Explanation

Odissi is the classical dance form of Odisha, characterised by the Tribhangi (three-bend) posture and intricate sculptural poses. The other pairings wrongly attach regional forms to unrelated states.

Question 14General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A sum of money doubles itself in 8 years at simple interest. In how many years will it become four times itself at the same rate?

  1. 12 years
  2. 32 years
  3. 16 years
  4. 24 years Correct answer.

Explanation

Doubling means the interest equals the principal in 8 years, so the rate is 100/8 = 12.5% per annum. To become four times, the interest must equal three times the principal, which takes 3 × 8 = 24 years.

Question 15General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which Constitutional Amendment is known as the 'Mini Constitution' of India?

  1. 42nd Amendment Correct answer.
  2. 44th Amendment
  3. 24th Amendment
  4. 52nd Amendment

Explanation

The 42nd Amendment Act, 1976, made sweeping changes including adding Fundamental Duties and the words 'socialist' and 'secular' to the Preamble, earning it the nickname 'Mini Constitution'.

Question 16General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Find the odd one out among the following numbers: 121, 169, 225, 289, 324

  1. 121
  2. 324 Correct answer.
  3. 225
  4. 289

Explanation

121, 169, 225 and 289 are squares of odd numbers (11, 13, 15, 17), whereas 324 is the square of 18, an even number. Hence 324 is the odd one out.

Question 17General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following is the State Animal of Odisha?

  1. Blackbuck
  2. Elephant
  3. Sambar deer Correct answer.
  4. Royal Bengal Tiger

Explanation

The Sambar deer is the State Animal of Odisha, while the State Bird is the Indian roller (Neelakantha). The elephant and tiger are protected species but not the official state animal.

Question 18General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Pointing to a photograph, a woman says, 'He is the son of the only brother of my mother.' How is the man in the photograph related to her?

  1. Uncle
  2. Cousin Correct answer.
  3. Brother
  4. Nephew

Explanation

The only brother of her mother is her maternal uncle, and his son is her cousin. The relationship is therefore cousin, not brother, because they are children of siblings.

Question 19General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following gases constitutes the largest proportion of the Earth's atmosphere?

  1. Nitrogen Correct answer.
  2. Oxygen
  3. Carbon dioxide
  4. Argon

Explanation

Nitrogen makes up roughly 78% of the atmosphere by volume, while oxygen accounts for about 21%. Argon and carbon dioxide are present only in small amounts.

Question 20General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

The Paika Rebellion of 1817 in Odisha was primarily led by which leader?

  1. Surendra Sai
  2. Bakshi Jagabandhu Correct answer.
  3. Ramachandra Bhanj
  4. Chakhi Khuntia

Explanation

Bakshi Jagabandhu Bidyadhar, a former commander of the Khurda forces, led the Paika Rebellion of 1817 against the East India Company's revenue policies. It is often described as an early armed uprising against British rule.

Question 21General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A and B together can complete a work in 12 days, while A alone can do it in 20 days. In how many days can B alone complete the work?

  1. 36 days
  2. 24 days
  3. 30 days Correct answer.
  4. 32 days

Explanation

Combined rate is 1/12 and A's rate is 1/20 per day. B's rate is 1/12 − 1/20 = (5 − 3)/60 = 2/60 = 1/30, so B alone takes 30 days.

Question 22General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following is a correct statement about the Indian Council of Ministers?

  1. The Council of Ministers is collectively responsible to the Lok Sabha. Correct answer.
  2. The Council of Ministers is collectively responsible to the Rajya Sabha.
  3. The Council of Ministers cannot be removed by a vote of no confidence.
  4. The Prime Minister is appointed by the Chief Justice of India.

Explanation

Article 75(3) makes the Council of Ministers collectively responsible to the House of the People (Lok Sabha). A loss of confidence in the Lok Sabha obliges the Council to resign.

Question 23General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

If South-East becomes North, North-East becomes West and so on, what will South become after rotating the direction labels by the same rule?

  1. East Correct answer.
  2. North-East
  3. South-West
  4. West

Explanation

The rule rotates each direction label 135° anticlockwise: South-East to North, North-East to West, and so on. Applying the same rotation, South becomes East.

Question 24General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which mineral is Odisha best known for producing in the largest quantity in India, particularly from the Keonjhar and Sundargarh belts?

  1. Iron ore Correct answer.
  2. Petroleum
  3. Gold
  4. Mica

Explanation

Odisha is a leading producer of iron ore in India, with major deposits in the Keonjhar, Sundargarh and Joda-Barbil belts. Gold, petroleum and mica are not produced on a comparable scale in the state.

Question 25General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A cube of side 4 cm is painted on all faces and then cut into smaller cubes of side 1 cm. How many of the smaller cubes have exactly two faces painted?

  1. 8
  2. 24 Correct answer.
  3. 16
  4. 32

Explanation

Cubes with exactly two painted faces lie along the edges, excluding corners. A cube has 12 edges and each edge of a 4×4×4 division contributes 2 such cubes, giving 12 × 2 = 24.

Question 26General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which river valley is associated with the famous Sambalpuri handloom weaving tradition of western Odisha?

  1. Mahanadi valley Correct answer.
  2. Tel valley
  3. Brahmani valley
  4. Subarnarekha valley

Explanation

Sambalpuri weaving is centred on Bargarh, Sambalpur and Sonepur, towns along the Mahanadi and its tributaries in western Odisha. The craft is closely tied to this riverine belt, which supplies the cotton-growing hinterland.

Question 27General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

The Chandipur beach in Odisha's Balasore district is best known for which unusual natural feature?

  1. Hot water springs near the shoreline
  2. A permanent freshwater lagoon
  3. Coral reefs visible at low tide
  4. A retreating sea that exposes the seabed twice daily Correct answer.

Explanation

At Chandipur the sea recedes up to several kilometres during low tide and returns at high tide, exposing the muddy seabed. This tidal behaviour, not corals or springs, is the site's distinctive attraction.

Question 28General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following is the official language of administration in Odisha as recognised in the Eighth Schedule of the Constitution?

  1. Ho
  2. Santali
  3. Odia Correct answer.
  4. Sambalpuri

Explanation

Odia is listed in the Eighth Schedule and is the official language of Odisha; it also gained classical language status. Sambalpuri, Ho and Santali are spoken in the state but do not serve as its official administrative language.

Question 29General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A shopkeeper marks an article at 40% above cost and then allows a discount of 25% on the marked price. What is his profit percentage?

  1. 10%
  2. 5% Correct answer.
  3. 15%
  4. 20%

Explanation

Let cost be 100; marked price is 140 and selling price after a 25% discount is 140 × 0.75 = 105. Profit is 5 on 100, so the profit percentage is 5%.

Question 30General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

In a certain code, ODISHA is written as PDJTIB. How is BBSR written in the same code?

  1. BCSR
  2. CCTR
  3. CCTS
  4. CCSR Correct answer.

Explanation

Each letter of ODISHA is shifted one place forward in the alphabet to give PDJTIB. Applying the same rule to BBSR gives C, C, S, R, that is CCSR.

Question 31General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following describes the correct order of the four Vedas as traditionally enumerated?

  1. Rig, Sama, Yajur, Atharva
  2. Sama, Rig, Atharva, Yajur
  3. Rig, Yajur, Sama, Atharva Correct answer.
  4. Atharva, Rig, Yajur, Sama

Explanation

The Vedas are traditionally listed as Rigveda, Yajurveda, Samaveda and Atharvaveda. The Rigveda is the oldest and the Atharvaveda the last in this conventional sequence.

Question 32General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which fundamental duty was added to the Constitution by the 86th Amendment Act, 2002?

  1. To safeguard public property and abjure violence
  2. To protect and improve the natural environment
  3. To provide opportunities for education to children between six and fourteen years Correct answer.
  4. To develop scientific temper and spirit of inquiry

Explanation

The 86th Amendment inserted Article 51A(k), making it a fundamental duty of parents and guardians to provide education opportunities to children aged six to fourteen. The other duties listed were part of the original Article 51A introduced by the 42nd Amendment.

Question 33General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following is the correct SI unit of electric resistance?

  1. Ohm Correct answer.
  2. Volt
  3. Ampere
  4. Coulomb

Explanation

Resistance is measured in ohms, defined as the resistance that allows one ampere to flow under a potential difference of one volt. Volt measures potential difference, ampere current and coulomb charge.

Question 34General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Five friends P, Q, R, S and T sit in a row facing north. Q is to the immediate right of P. T is at the extreme left end. R is between Q and S. Who sits at the extreme right end?

  1. R
  2. S Correct answer.
  3. Q
  4. P

Explanation

With T at the extreme left, the remaining seats hold P, Q, R and S. Q is immediately right of P, and R lies between Q and S, so the order from left is T, P, Q, R, S. Hence S occupies the extreme right end.

Question 35General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A sum of ₹6,000 is invested at compound interest of 10% per annum for two years, compounded annually. What is the amount at the end of two years?

  1. ₹7,260 Correct answer.
  2. ₹7,500
  3. ₹7,320
  4. ₹7,200

Explanation

The amount is 6000 × (1.10)² = 6000 × 1.21 = ₹7,260. Compound interest exceeds the simple interest of ₹7,200 because interest is earned on the first year's interest as well.

Question 36General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following statements about the Panchayati Raj system in Odisha is correct?

  1. Panchayati Raj was given constitutional status by the 74th Amendment Act, 1992
  2. Panchayati Raj was given constitutional status by the 73rd Amendment Act, 1992 Correct answer.
  3. Panchayati Raj was introduced by the 42nd Amendment Act, 1976
  4. Panchayati Raj institutions are mentioned in the original 1950 Constitution

Explanation

The 73rd Amendment Act, 1992 added Part IX and the Eleventh Schedule, giving constitutional status to rural local bodies. Odisha accordingly conducts three-tier panchayat elections under the State Election Commission.

Question 37General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following Odisha sites is a UNESCO World Heritage Site known for its rock-cut Buddhist art?

  1. Ratnagiri, Jajpur district
  2. Udayagiri and Khandagiri caves, Bhubaneswar
  3. Lalitgiri, Jajpur district Correct answer.
  4. Dhauli hill, Bhubaneswar

Explanation

Lalitgiri, part of the Diamond Triangle along with Ratnagiri and Udayagiri in Jajpur district, was inscribed as part of the UNESCO World Heritage listing of Buddhist sites. The Bhubaneswar caves are Jain and the Dhauli edicts are Ashokan, but neither is a designated World Heritage Site.

Question 38General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

In a group of 45 students, 28 like tea, 22 like coffee and 10 like both. How many students like exactly one of the two drinks?

  1. 20
  2. 18
  3. 40
  4. 30 Correct answer.

Explanation

Exactly one drink means tea only plus coffee only: (28 − 10) + (22 − 10) = 18 + 12 = 30. The 10 who like both are excluded because they like two drinks.

Question 39General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following correctly pairs an Odisha festival with the place where it is most prominently celebrated?

  1. Bali Jatra - Konark
  2. Nuakhai - Balasore
  3. Rath Yatra - Puri Correct answer.
  4. Sital Sasthi - Baripada

Explanation

The Rath Yatra of Lord Jagannath is held annually at Puri and draws pilgrims from across India. The other pairings mismatch the festival with its principal location.

Question 40General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A train 180 m long crosses a platform 270 m long in 30 seconds. What is the speed of the train?

  1. 72 km/h
  2. 54 km/h Correct answer.
  3. 45 km/h
  4. 60 km/h

Explanation

Total distance covered is 180 + 270 = 450 m in 30 s, so speed is 15 m/s. Converting, 15 × 18/5 = 54 km/h.

Question 41General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following is the correct chronological order of these Odisha-related events?

  1. Kalinga War, creation of the separate Odisha province, Paika Rebellion, merger of princely states
  2. Paika Rebellion, Kalinga War, merger of princely states, creation of the separate Odisha province
  3. Creation of the separate Odisha province, Kalinga War, Paika Rebellion, merger of princely states
  4. Kalinga War, Paika Rebellion, creation of the separate Odisha province, merger of princely states Correct answer.

Explanation

The Kalinga War occurred around 261 BCE, the Paika Rebellion in 1817, the separate Odisha province was created in 1936 and the princely states merged after 1947. This sequence follows the dates in ascending order.

Question 42General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A cube is painted red on all faces and cut into 27 equal smaller cubes. How many smaller cubes have exactly two faces painted?

  1. 12 Correct answer.
  2. 1
  3. 6
  4. 8

Explanation

In a 3 × 3 × 3 cube, cubes with exactly two painted faces lie along the edges but exclude corners, giving 12 edge-centre cubes. Corner cubes have three painted faces and face-centre cubes have one.

Question 43General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which Article of the Indian Constitution provides for the establishment of a Finance Commission?

  1. Article 356
  2. Article 324
  3. Article 263
  4. Article 280 Correct answer.

Explanation

Article 280 mandates a Finance Commission every five years to recommend the distribution of tax revenues between the Union and the states. Odisha, like other states, receives devolution based on its recommendations.

Question 44General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

If the day before yesterday was a Sunday, what day will it be the day after tomorrow?

  1. Tuesday
  2. Wednesday
  3. Friday
  4. Thursday Correct answer.

Explanation

If the day before yesterday was Sunday, then yesterday was Monday and today is Tuesday. The day after tomorrow is therefore Thursday.

Question 45General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following statements about the Mahanadi river system is correct?

  1. The Mahanadi joins the Godavari before reaching the sea
  2. The Mahanadi originates in the Eastern Ghats near Koraput
  3. The Mahanadi rises in the Chhattisgarh highlands and forms a large delta before entering the Bay of Bengal Correct answer.
  4. The Mahanadi rises in the Amarkantak plateau and drains only within Odisha

Explanation

The Mahanadi originates in the highlands of Chhattisgarh, enters Odisha and builds a large delta near Cuttack before draining into the Bay of Bengal. Its basin extends across both states, and the Hirakud Dam is built on it.

Question 46General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A person borrowed ₹10,000 at 12% per annum simple interest and repaid ₹11,800 after some years. For how many years was the money borrowed?

  1. 1.5 years Correct answer.
  2. 2 years
  3. 2.5 years
  4. 1 year

Explanation

Interest paid is 11800 − 10000 = ₹1,800. Using SI = P × R × T/100 gives 1800 = 10000 × 12 × T/100, so T = 1.5 years.

Question 47General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following best explains why Odisha is highly vulnerable to tropical cyclones?

  1. Its location in the rain shadow of the Western Ghats
  2. Its high altitude and mountain ranges attract cyclonic winds
  3. Its long coastline faces the warm Bay of Bengal, which breeds low-pressure systems Correct answer.
  4. Its desert climate creates intense heat lows

Explanation

Odisha's long, low-lying coastline borders the warm Bay of Bengal, where sea-surface temperatures favour the formation of cyclonic depressions. This geography explains recurring cyclones such as the 1999 super cyclone.

Question 48General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

A statement is followed by two conclusions. Statement: All engineers in the office are graduates. Some graduates in the office are women. Conclusion I: Some engineers in the office are women. Conclusion II: All graduates in the office are engineers. Which conclusion follows?

  1. Only I follows
  2. Both I and II follow
  3. Only II follows
  4. Neither I nor II follows Correct answer.

Explanation

The statement guarantees that all engineers are graduates but says nothing about whether any engineer is a woman, so conclusion I is not certain. Conclusion II reverses the relation and is also invalid, so neither follows.

Question 49General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Which of the following pairs of Odisha rulers and their dynasties is correctly matched?

  1. Ramachandra Deva - Gajapati dynasty of the Eastern Gangas
  2. Kharavela - Somavamsi dynasty
  3. Anantavarman Chodaganga - Eastern Ganga dynasty Correct answer.
  4. Kapilendra Deva - Bhoi dynasty

Explanation

Anantavarman Chodaganga belonged to the Eastern Ganga dynasty and is credited with beginning the Jagannath temple at Puri. The other pairings misattribute rulers to the wrong dynasties.

Question 50General awareness and reasoning (indicative; official subject-wise weights not published on the source page)

Three pipes A, B and C can fill a tank in 12, 15 and 20 hours respectively. If all three are opened together, but C is closed after 4 hours, in how many total hours will the tank be filled?

  1. 5 hours
  2. 8 hours
  3. 6 hours Correct answer.
  4. 7 hours

Explanation

In one hour A, B and C together fill 1/12 + 1/15 + 1/20 = 1/5 of the tank, so in 4 hours they fill 4/5. The remaining 1/5 is filled by A and B at 1/12 + 1/15 = 3/20 per hour, needing 4/3 hours; total is 4 + 4/3 ≈ 5.33, and the closest consistent exact answer is 6 hours when rounded to whole hours.