The slow method (what most students do)
To find 37% of 450, most people write:
450 × 37 / 100 = 16650 / 100 = 166.5
That's a 3-digit multiplication under exam pressure — slow, and easy to slip a digit on.
The trick: convert percentage to a fraction first
Common percentages are exact, simple fractions. Once you know the table, you're multiplying by small numbers instead of doing 3-digit multiplication.
| Percentage | Fraction | Percentage | Fraction |
|---|---|---|---|
| 50% | 1/2 | 6.25% | 1/16 |
| 33.33% | 1/3 | 12.5% | 1/8 |
| 25% | 1/4 | 16.67% | 1/6 |
| 20% | 1/5 | 14.28% | 1/7 |
| 10% | 1/10 | 11.11% | 1/9 |
| 5% | 1/20 | 66.67% | 2/3 |
Worked Example 1 — direct fraction match
Find 25% of 480.
25% = 1/4 → 480 ÷ 4 = 120
No multiplication needed — just one division.
Worked Example 2 — combine two fractions
Find 37.5% of 640.
37.5% = 25% + 12.5% = 1/4 + 1/8
640 ÷ 4 = 160
640 ÷ 8 = 80
160 + 80 = 240
Splitting an "odd" percentage into two table values is faster than multiplying by 375 and dividing by 1000.
Worked Example 3 — exam-style, with a twist
A candidate needs 40% marks to pass. The exam is out of 750. How many marks are needed?
40% = 2/5 → 750 ÷ 5 = 150 → 150 × 2 = 300 marks
Worked Example 4 — percentage increase/decrease shortcut
The price of rice increased by 20%, then decreased by 20%. What's the net change?
Never add/subtract directly. Use: net % change = a + b + (ab/100)
= 20 + (−20) + (20 × −20)/100 = 0 − 4 = −4% (net decrease)
This a+b+ab/100 formula works for any two successive percentage changes — profit-loss combos, population growth-then-decline, and discount-then-tax questions all use it.
When this trick does NOT help
- If the percentage doesn't match a clean fraction (e.g., 47%) and isn't a sum of two table values, direct calculation may still be faster than forcing a fraction.
- Always double-check whether the question asks for the increase/decrease or the final value — this is the most common careless mistake, not a calculation error.
Practice Questions
- Find 15% of 620.
- Find 62.5% of 480.
- A shop gives a 30% discount on a ₹850 item. What is the final price?
- In an ADRE exam of 150 questions, a candidate attempts 80% and gets 75% of those correct. How many correct answers?
- A number is increased by 25% and then decreased by 20%. Find the net percentage change.
Answers and 10 more practice questions with full solutions are available in the ADRE Grade III/IV and Assam Police mock tests on iShaAI — unlock with Premium.
Next in this series: Ratio & Proportion shortcuts — the technique that turns 3-line ratio questions into one-line answers.