Arithmetical & Numerical Ability for DSSSB PRT
Twenty questions test speed and accuracy on school-level arithmetic: simplification and BODMAS, number system, HCF–LCM, percentage, ratio, average, profit and loss, discount, simple and compound interest, time and work, speed–distance–time, mensuration, and data interpretation from tables, bar graphs or pie charts.
Formulas that earn marks
- Successive percentage change: a + b + ab/100 (so 10% then 10% = 19%, not 20%).
- Profit% and loss% are always calculated on cost price; discount is always on marked price.
- SI = PRT/100; CI = P(1 + r/100)^t − P; for two years the CI–SI gap is P(r/100)².
- Work: add rates (1/A + 1/B); efficiency is inversely proportional to days taken.
- Speed: 1 m/s = 18/5 km/h; average speed = total distance ÷ total time, never the mean of speeds.
Worked example
A shopkeeper marks an article 40% above cost and allows a 25% discount. Find the profit percentage. Step 1: let CP = 100, so MP = 140. Step 2: discount is on MP: 25% of 140 = 35, so SP = 140 − 35 = 105. Step 3: profit = 105 − 100 = 5 on CP of 100, i.e. 5%. The discount must never be taken on CP; doing that gives 40 − 25 = 15%, a classic wrong option.
Traps that cost marks
- Adding successive discounts instead of compounding them.
- Computing profit% on selling price.
- Using the arithmetic mean for average speed on a two-part journey.
- Forgetting unit conversions in mensuration (cm to m) and in speed questions.
- Mishandling BODMAS: brackets, then orders, division/multiplication left to right, then addition/subtraction.
- In data interpretation, answering from a percentage when the question asks for an absolute value.