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Mental math for trading tests: the core tricks
Break-and-rebuild, round-number tricks, fractions to know cold and decimal placement: the methods that make timed arithmetic tests fast and accurate.
الأدلة باللغة الإنجليزية، لغة المقابلات.
Why speed comes from method
Timed arithmetic tests are common at trading firms, often dozens of questions in a few minutes. The fastest candidates don't calculate harder: they recognize the shape of each question and apply a trick they've practiced.
The tricks
- 1
Break and rebuild
Split one number into easy parts: 47 × 6 = 40 × 6 + 7 × 6 = 240 + 42.
- 2
Use a nearby round number
98 × 7 = 700 − 14. For numbers either side of a round one, use a difference of squares: 49 × 51 = 50² − 1.
- 3
Squares ending in 5
Take the leading digit n, multiply by n + 1, and write 25 after it: 65² → 6 × 7 = 42 → 4,225.
- 4
Multiply by 5, 25 or 125
× 5 is × 10 ÷ 2; × 25 is × 100 ÷ 4; × 125 is × 1,000 ÷ 8.
- 5
Flip percentages
x% of y = y% of x: 8% of 25 = 25% of 8 = 2. Build others from 10% and 5%: 15% of 80 = 8 + 4.
- 6
Clear the decimal before dividing
252 ÷ 1.2 = 2,520 ÷ 12.
- 7
Count decimal places
0.3 × 0.04: 3 × 4 = 12, with three decimal places in total, so 0.012.
| Fraction | Decimal | Fraction | Decimal |
|---|---|---|---|
| 1/8 | 0.125 | 1/6 | 0.1667 |
| 3/8 | 0.375 | 1/7 | 0.1429 |
| 5/8 | 0.625 | 1/9 | 0.1111 |
| 7/8 | 0.875 | 1/12 | 0.0833 |
Know these cold: they turn many divisions into a lookup.
Worked examples
مثال محلول · Five quick ones
تم التحقق برمجيًا47 × 6, 49 × 51, 65², 36 × 25 and 15% of 80.
اعرض خطوات الحل
- 1.47 × 6 = 240 + 42 = 282.
- 2.49 × 51 = 2,500 − 1 = 2,499.
- 3.65² = 42 then 25 = 4,225.
- 4.36 × 25 = 3,600 ÷ 4 = 900.
- 5.15% of 80 = 8 + 4 = 12.
الإجابة282, 2,499, 4,225, 900 and 12.
مثال محلول · Decimals and division
تم التحقق برمجيًا252 ÷ 1.2 and 0.3 × 0.04.
اعرض خطوات الحل
- 1.252 ÷ 1.2 = 2,520 ÷ 12 = 210.
- 2.3 × 4 = 12 with three decimal places: 0.012.
الإجابة210 and 0.012.
نصيحةBefore you answer, check the size and the last digit. 47 × 6 must end in 2 and sit near 50 × 6 = 300.
Common mistakes
أخطاء شائعة
- Misplacing the decimal point under time pressure.
- Getting stuck on one hard question. Move on and come back.
- Guessing blindly when wrong answers cost marks. Read the test's rules first.
- Practicing only untimed. Speed is a separate skill: train with the clock running.
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