Mathematics
Mathematics in the VDO paper is basic numeracy at Class 8–10 level. Arithmetic - percentage, ratio, average, profit and loss, interest, time and work, speed and distance - supplies most questions, with a few on mensuration and simplification.
Rules worth memorising
- Successive percentage change: a + b + ab/100, using signs (a discount is negative).
- Profit% and loss% are always calculated on cost price unless stated otherwise: SP = CP × (100 + profit%)/100.
- CI = P[(1 + r/100)^t − 1]; for two years the CI − SI difference is P(r/100)². For half-yearly compounding, halve the rate and double the time.
- Time and work: take the LCM of the given days as total work in units, then convert every worker into units per day.
- Speed: km/h to m/s multiply by 5/18; a train crossing a platform covers train length plus platform length.
Worked example
A finishes a job in 12 days and B in 18 days. They work together for 4 days, then A leaves. How long does B take to finish the rest? Reasoning: LCM(12, 18) = 36 units of work. A does 3 units/day, B does 2, together 5. In 4 days they finish 20 units, leaving 16. B alone needs 16 ÷ 2 = 8 days. Total time = 4 + 8 = 12 days. The unit method avoids fractions and is faster than building equations.
Traps
- Adding successive discounts instead of applying them one after another.
- Computing profit% on selling price rather than cost price.
- Forgetting to convert minutes to hours in speed questions.
- Mixing radius with diameter, or using 22/7 when options are decimals.
Question shape
Expect one simplification, one discount, one interest, and one work or speed question.