Mental Math for Investment Banking Interviews: Drills and Tricks

Nobody gets an IB offer for being a human calculator, but plenty of candidates lose momentum in interviews by fumbling arithmetic they would nail on paper. Mental math questions in banking interviews are rarely hard math; they are simple operations under time pressure with someone watching. That is a trainable skill.

This guide covers the four things that actually get tested — percentages, multiples, growth rates, and paper-LBO arithmetic — plus the small set of tricks worth memorizing and a drill routine to make them automatic.

What interviewers actually test

The math shows up embedded in technical questions rather than as standalone puzzles. You will be mid-answer on a valuation question and suddenly need a multiple or a percentage. The four recurring flavors:

• Percentages: margins, premiums, discounts. What is the EBITDA margin if EBITDA is 60 on revenue of 400? Answer: 15 percent.
• Multiples: divide two round numbers. EV of 500 on EBITDA of 50 is 10.0x; a company earning 200 of net income valued at 3,000 of equity trades at 15x earnings.
• Growth and compounding: revenue growing 20 percent for two years is up 44 percent cumulatively, because 1.2 times 1.2 equals 1.44, not 1.40.
• Paper-LBO arithmetic: multiply an EBITDA by an entry multiple, apply a tax rate, sum five years of cash flow, subtract remaining debt, and turn a money multiple into a rough IRR.

Percentage tricks: anchor on 10 percent and 1 percent

Almost every percentage question falls quickly once you anchor on an easy reference. Ten percent is just moving the decimal; from there you scale. Fifteen percent of 400 is 10 percent (40) plus half of that (20), so 60. Four percent of 250 is 1 percent (2.5) times four, so 10.

Two specific patterns are worth internalizing. First, a percentage of a number equals that number as a percentage of the original swapped: 8 percent of 50 equals 50 percent of 8, which is 4 — often one direction is much easier. Second, percentage changes are not symmetric: if a stock falls 20 percent, it must rise 25 percent to get back to even, because 0.8 times 1.25 equals 1.0. Interviewers love that asymmetry.

The rule of 72 and its cousins

The rule of 72 estimates how long an investment takes to double: divide 72 by the annual growth rate in percent. At 8 percent, doubling takes about 72 divided by 8, or 9 years. At 6 percent, about 12 years. It also runs in reverse: if something doubled in 8 years, it compounded at roughly 9 percent per year.

It is an approximation — the exact doubling constant near typical rates is closer to 69 or 70, and 72 is used because it divides cleanly by 2, 3, 4, 6, 8, 9, and 12. The approximation is tight for rates in the mid single digits to low teens, which covers nearly everything you will see in an interview. Sanity check: 1.09 compounded for 8 years is approximately 1.99, essentially a double.

Multiples and growth without a calculator

For multiples, strip the zeros and simplify the fraction. EV of 4,200 on EBITDA of 600 is 42 over 6, so 7.0x. If the division is ugly, bracket it: 42 over 6.5 is a bit under 6.5x, and saying roughly six and a half times is a perfectly good interview answer when the inputs are round to begin with.

For compounding, remember that small rates compound almost linearly and bigger rates do not. Growing 5 percent for three years is about 15.8 percent cumulative (1.05 cubed is roughly 1.158), close to the naive 15. Growing 20 percent for three years is 72.8 percent cumulative (1.2 cubed is 1.728), far from the naive 60. When precision matters, add the simple sum plus a compounding kicker; when it does not, say approximately and move on.

Paper-LBO arithmetic and the MoM-to-IRR shortcuts

Paper LBOs stress two skills: keeping a running tally without dropping numbers, and converting a money-on-money multiple into an approximate IRR at the end. The tally skill is pure practice. The conversion is a lookup table worth memorizing for a five-year hold:

These follow from compounding math: 1.15 to the fifth power is approximately 2.01, and 1.25 to the fifth is approximately 3.05. Interviewers accept the approximations; they are testing whether you know the mapping at all. For other holds, the rule of 72 gets you close: 2x in 5 years means doubling, so roughly 72 divided by 5, about 14 to 15 percent.

• 2.0x over 5 years: roughly 15 percent IRR
• 3.0x over 5 years: roughly 25 percent IRR
• 4.0x over 5 years: roughly 32 percent IRR

A 10-minute daily drill routine

Speed comes from short, frequent, slightly uncomfortable practice, not marathon sessions. Ten minutes a day for two weeks transforms most candidates. Do the reps out loud, because saying numbers while thinking is its own skill and it is exactly what the interview demands.

If you want structure, WACC Buddy's timed drills put these operations on a clock so the pressure in the interview is not the first pressure you have felt.

1. 01Minutes 1-3: ten percentage questions (margins, premiums, the down-X-up-Y asymmetry)
2. 02Minutes 4-6: ten multiples, dividing round numbers and bracketing ugly ones
3. 03Minutes 7-8: five compounding questions, including one rule-of-72 in each direction
4. 04Minutes 9-10: one mini paper-LBO tally, then convert the multiple to an IRR from the table

Under pressure: narrate, round, sanity-check

In the room, three habits save you. Narrate your steps so silence never becomes awkward and the interviewer can follow your logic even if you slip. Round aggressively and say so — call 9.6 percent roughly 10 percent, compute, then adjust the answer down a touch. And sanity-check the magnitude before you commit: if a company with 100 of EBITDA at a 10x multiple came out to 10,000 in your head, you dropped a decimal, and catching it yourself is a strong signal rather than a weakness.

FAQ