Percentage shortcuts for competitive exams
Percentage questions show up everywhere in SSC, RRB, and Bank exams — often disguised inside profit/loss, data interpretation, or simple/compound interest questions. Two habits make all of them faster.
Habit one: know the fraction equivalents cold
50% = 1/2 25% = 1/4 20% = 1/5
12.5% = 1/8 10% = 1/10 33.33% = 1/3
16.67% = 1/6 6.25% = 1/16
Multiplying by a clean fraction is almost always faster than multiplying by a decimal. 25% of 84 is instantly 84 ÷ 4 = 21 — no decimal multiplication needed.
Habit two: build any percentage from 10% and 1%
10% of a number is just moving the decimal one place. 1% is moving it two places. Every other percentage is some combination of those two building blocks.
Example: 23% of 450. 10% = 45. So 20% = 90. 1% = 4.5, so 3% = 13.5. Add: 90 + 13.5 = 103.5.
Worked example: 17.5% of 640
10% = 64. 5% = half of that = 32. 2.5% = half again = 16. Add 10% + 5% + 2.5% = 64 + 32 + 16 = 112.
Successive percentage change
A common exam trap: a 20% increase followed by a 20% decrease does not cancel out. The shortcut formula for two successive percentage changes a% and b%:
For +20% then −20%: 20 + (−20) + (20×−20)/100 = 0 − 4 = −4% net. The end result is a 4% decrease, not zero — the classic trap this formula catches instantly.