Mental Math Multiplication Shortcuts for Interviews
Mental math multiplication shortcuts for quant interviews, including decomposition, doubling and halving, rounding, error checks, and drill design.
Candidates who need faster two-digit and rough multiplication under pressure.
Decompose before multiplying
Break awkward products into easier parts. Multiplying 47 by 12 is 47 by 10 plus 47 by 2, which is often safer than trying to hold the full product at once.
Use doubling and halving
When one factor is even, halve it and double the other factor if that creates a friendlier product. For example, 25 x 48 becomes 50 x 24 or 100 x 12.
Concrete example
For 38 x 15, use 38 x 10 plus 38 x 5: 380 + 190 = 570. A quick check says 40 x 15 is 600, so 570 is plausible and the direction of the correction is right.
Round then correct
Round one factor to a nearby easy value, multiply, then subtract the correction. This works well when the correction is small and easy to calculate.
Common mistakes
Candidates often use a shortcut but skip the sanity check. Always compare with a rough product so a misplaced zero or sign error is caught quickly.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.