Short answer
Round each number to the nearest easy value, turn percentages into fractions, use the nearest perfect square for roots, and decide how precise to be by looking at how far apart the options are.
Approximation questions are built to be rounded. Numbers like 24.98% or 639.9 are there so you round them, not calculate them. The skill is knowing how much rounding is safe.
Five rules
- Round to the nearest easy number: 639.9 → 640, 15.01 → 15, 7.99 → 8.
- Percentages as fractions: 24.98% → 1/4, 33.3% → 1/3, 12.4% → 1/8.
- Roots to perfect squares and cubes: √1299 → 36 (36² = 1296), ∛730 → 9 (9³ = 729).
- Balance rounding in products: if you round one number up, try to round the other down.
- Check the option gap: if options differ by 20 or more, aggressive rounding is fine. If they differ by 2-3, keep one decimal.
Worked examples
| Question | Rounded | Answer |
|---|---|---|
| 24.98% of 639.9 + 15.01 × 7.99 | 640 ÷ 4 + 15 × 8 | 160 + 120 = 280 |
| 449.89 ÷ 15.02 + √1299 | 450 ÷ 15 + 36 | 30 + 36 = 66 |
| (7.99)² − 3.01³ | 64 − 27 | 37 |
| 12.4% of 799.7 − 39.98 | 800 ÷ 8 − 40 | 100 − 40 = 60 |
"Find the missing value" type
When the question is "? × 14.98 ≈ 449.8", round both sides first: ? × 15 = 450, so ? = 30. Do not solve with the decimals.
Practice
Do 20 approximation questions daily for two weeks with a timer. Target under 30 seconds each. After that, mix them into your simplification drill.
Common mistakes
- Rounding a divisor too far: 449 ÷ 14.6 is closer to 449 ÷ 15 than to 449 ÷ 14, but the gap matters if options are close.
- Rounding a squared number: 7.6² is about 58, not 64.
- Forgetting BODMAS while rounding: do division and multiplication before addition and subtraction.
Approximation and simplification together often give 10 or more Prelims questions. They are the fastest marks in Quant, so attempt them first.
Sources
Rules can change with each notification. Confirm on sbi.bank.in/web/careers.
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