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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.
Les guides sont en anglais, la langue des entretiens.
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
Exemple corrigé · Five quick ones
Vérifié par code47 × 6, 49 × 51, 65², 36 × 25 and 15% of 80.
Voir la résolution
- 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.
Réponse282, 2,499, 4,225, 900 and 12.
Exemple corrigé · Decimals and division
Vérifié par code252 ÷ 1.2 and 0.3 × 0.04.
Voir la résolution
- 1.252 ÷ 1.2 = 2,520 ÷ 12 = 210.
- 2.3 × 4 = 12 with three decimal places: 0.012.
Réponse210 and 0.012.
ConseilBefore you answer, check the size and the last digit. 47 × 6 must end in 2 and sit near 50 × 6 = 300.
Common mistakes
Erreurs fréquentes
- 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.
S’entraîner
Appliquez la méthode à de nouvelles questions, corrigées instantanément.
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