Mathematics / Numerical Ability for TNUSRB Constable
Numerical Ability tests simplification, percentage, ratio, average, profit and loss, simple and compound interest, time and work, time, speed and distance, and number series. The paper rewards clean shortcuts and careful reading more than long calculation.
Rules to keep ready
- Percentage change = (change ÷ original) × 100; the base is always the original value.
- Successive discounts of a% and b% give a net discount of a + b − ab/100, not a + b.
- For two equal distances at speeds x and y, average speed = 2xy/(x+y).
- Difference between CI and SI for 2 years = P(r/100)².
- If A alone finishes work in x days and B in y days, together they take xy/(x+y) days.
Worked example
A shirt marked ₹1,200 is sold after successive discounts of 20% and 10%. Find the selling price. First discount = 20% of 1,200 = ₹240, leaving ₹960. The second discount applies to ₹960, not ₹1,200: 10% of 960 = ₹96. Selling price = ₹864. Shortcut check: net discount = 20 + 10 − (20×10)/100 = 28%, so 1,200 × 0.72 = ₹864.
Traps that cost marks
Applying both discounts to the marked price is the commonest error. In average-speed questions, use (x+y)/2 only when the times are equal, never when the distances are equal. In interest, remember SI stays the same each year while CI grows. In ratio problems, convert everything to the same unit before forming the ratio, and re-read whether the question asks for profit percent on cost price or on selling price.