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Brainteasers

Mental math for the interview

By SuperdayReps · Wharton MBA · ex-Greenhill · investment banking interview coaching

6 min read · updated July 2, 2026

You will not get a calculator. Someone will slide a number across the table, ask you to double it, take a percent of it, or turn a 3x return over five years into a rate, and they will watch your face while you do it. The math is never the hard part. Your composure is. A banker who freezes on "what's 15% of 80" is telling the room they can't be trusted with a live deal at 11pm.

So train the reflexes. Not the theory, the reflexes. Here's the toolkit.

The rule of 72

This is the one shortcut you must own cold. To find how many years it takes something to double, divide 72 by the growth rate.

Years to double≈72rate (%)\text{Years to double} \approx \frac{72}{\text{rate }(\%)}

Money growing at 8% doubles in 9 years. At 12%, in 6 years. At 6%, in 12 years. It runs backwards too: if something doubled in 4 years, the rate was roughly 72 / 4 = 18%.

Key insight

The rule of 72 is really a shortcut for compounding, and compounding is the engine under every return question in finance. If you can double and halve fast, you can approximate almost any growth or discount problem in your head.

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Frequently asked

What is the rule of 72 and how do you use it?
The rule of 72 tells you how many years something takes to double: divide 72 by the growth rate. Money growing at 8% doubles in 9 years, at 12% in 6 years. It runs backwards too, so something that doubled in 4 years grew at roughly 18%.
What is the fastest way to calculate percentages in your head?
Break the percent into blocks you already know. 10% just moves the decimal, 5% is half of that, and 1% moves the decimal twice. Then build any percent from those pieces. For 15% of 340, add 10% which is 34 to 5% which is 17, giving 51.
How do you estimate IRR from MOIC without a calculator?
There is no formula to compute in your head, so you estimate using the rule of 72 and memorized anchors. Over a five-year hold, a 2x is mid-teens, a 2.5x is about 20%, and a 3x is mid-20s. Know those three and you can interpolate almost anything a paper LBO throws at you.
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