How do you multiply 48 by 52 in your head?
What the interviewer is testing
Comfort with numbers under time pressure — mental arithmetic comes up throughout trading interviews.
Answer it out loud firstTake 30–60 seconds, as you would in the room. Then compare with the model answer.
Show the model answer
Notice they are equally spaced around 50: 48 × 52 = (50 − 2)(50 + 2) = 50² − 2² = 2,500 − 4 = 2,496. In general, look for a round number to anchor on and use the difference of squares, splitting, or rounding and correcting.
The full answer
Useful tricks
| Trick | Example |
|---|---|
| Difference of squares | 48 × 52 = 50² − 2² = 2,496 |
| Split | 17 × 23 = 17 × 20 + 17 × 3 = 340 + 51 = 391 |
| Round and correct | 49 × 36 = 50 × 36 − 36 = 1,764 |
| Squares ending in 5 | 35²: 3 × 4 = 12, then 25 → 1,225 |
| Percentages | 15% of 360 = 10% (36) + 5% (18) = 54 |
| Basis points | 4 bp of €85m = 85m × 0.0004 = €34,000 |
Fractions worth knowing
1/8 = 12.5%, 1/7 ≈ 14.3%, 1/6 ≈ 16.7%, 1/9 ≈ 11.1%, 1/12 ≈ 8.3%.
Rule of 72
Doubling time ≈ 72 ÷ growth rate: at 8% about 9 years; €100 to €200 in 6 years ≈ 12% a year.
Mistakes to avoid
Going silent — talk through the method.
Follow-ups they may ask
How did your answer compare?
Log in to keep track of the questions you’ve mastered.