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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.
Die Leitfäden sind auf Englisch, der Sprache der Interviews.
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
Rechenbeispiel · Five quick ones
Per Code geprüft47 × 6, 49 × 51, 65², 36 × 25 and 15% of 80.
Lösungsweg anzeigen
- 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.
Antwort282, 2,499, 4,225, 900 and 12.
Rechenbeispiel · Decimals and division
Per Code geprüft252 ÷ 1.2 and 0.3 × 0.04.
Lösungsweg anzeigen
- 1.252 ÷ 1.2 = 2,520 ÷ 12 = 210.
- 2.3 × 4 = 12 with three decimal places: 0.012.
Antwort210 and 0.012.
TippBefore you answer, check the size and the last digit. 47 × 6 must end in 2 and sit near 50 × 6 = 300.
Common mistakes
Häufige Fehler
- 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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