Mental Math Drills for Investment Banking Interviews: How to Get Fast Without a Calculator

You're 20 minutes into a superday interview. The MD slides a blank sheet across the table and says, "Company does $100 million of EBITDA, we buy it at 10x, sell it at 10x in five years with $150 million of EBITDA. What's the IRR?"

No Excel. No calculator. Just you, a pen, and someone watching how you think.

This is where mental math drills pay off. Interviewers usually don't care if you hit the exact decimal. They care that you get close, fast, and calmly. This post gives you the specific skills to drill, the shortcuts that actually show up in interviews, and a simple daily routine to build speed.

Why Mental Math Matters in IB Interviews

Mental math isn't a party trick. It's a proxy for how you'll perform on the job.

When a VP asks "what's that as a multiple?" on a call, they expect a ballpark in seconds. Interviewers test this because:

• Paper LBOs require quick multiplication, division, and IRR estimates
• Valuation questions ask you to compute multiples, market caps, and enterprise values on the spot
• Brain teasers and market sizing need clean rounding and fast arithmetic
• Pressure reveals habits. Freezing on 15% of 80 says more than getting a hard question wrong

The good news: this is a trainable skill. A few weeks of focused drilling makes a visible difference.

The 6 Mental Math Skills Worth Drilling

Don't practice random arithmetic. Focus on the operations that keep showing up in finance questions.

1. Percentages (Your Bread and Butter)

You'll calculate margins, growth rates, and debt percentages constantly. A few tricks make this painless.

Break percentages into easy chunks:

• 10% = move the decimal one place
• 5% = half of 10%
• 1% = move the decimal two places

So 15% of 340 = 10% (34) + 5% (17) = 51.

Flip the percentage when it's easier: x% of y always equals y% of x.

• 18% of 50 is awkward
• 50% of 18 is 9
• Same answer: 9

Know the recovery asymmetry cold:

• Down 20% needs a 25% gain to get back to even
• Down 25% needs a 33% gain
• Down 50% needs a 100% gain

This comes up in markets questions more than you'd think.

2. Fractions to Decimals

Memorize these. They turn ugly divisions into instant answers.

Example: A company has $480 million in revenue and $60 million in EBITDA. What's the margin?

60/480 = 1/8 = 12.5%. Done in two seconds because you recognized the fraction.

3. Multiplication Shortcuts

Multiplying by 25: multiply by 100, then divide by 4.

• 36 × 25 = 3,600 / 4 = 900

Multiplying by 5: multiply by 10, then halve.

• 68 × 5 = 680 / 2 = 340

Numbers near a round number: use the difference of squares.

• 48 × 52 = (50 − 2)(50 + 2) = 2,500 − 4 = 2,496

Squaring numbers ending in 5: take the first digit, multiply it by the next number up, then add 25 to the end.

• 65²: 6 × 7 = 42, so 4,225
• 85²: 8 × 9 = 72, so 7,225

Break it apart: for anything messy, split into pieces.

• 7.5 × 120 = (7 × 120) + (0.5 × 120) = 840 + 60 = 900

4. Compounding and Growth

You need a feel for how numbers grow over time. Memorize a few anchors:

• 1.1² = 1.21
• 1.1³ = 1.331
• 1.1⁵ ≈ 1.61
• 1.2³ = 1.728
• 1.05⁵ ≈ 1.28

Example: Revenue is $200 million and grows 10% a year for three years. What's year-three revenue?

200 × 1.331 ≈ $266 million. No step-by-step compounding needed.

5. The Rule of 72 and IRR Shortcuts

The Rule of 72 says an investment doubles in roughly 72 ÷ (annual rate) years.

• At 8%, money doubles in about 9 years
• At 12%, about 6 years
• At 24%, about 3 years

Flip it to estimate IRR: if money doubles in 4 years, the IRR is roughly 72 / 4 = 18%.

For paper LBOs, memorize this cheat sheet of multiple of money (MoM) to approximate IRR:

If you know these six numbers, you can answer most paper LBO return questions instantly.

6. Valuation Arithmetic

Practice chaining steps together, since that's how interview questions actually work.

Example: Share price is $40, there are 50 million shares, debt is $500 million, cash is $100 million, and EBITDA is $300 million. What's the EV/EBITDA multiple?

1. Market cap: $40 × 50M = $2.0 billion
2. Enterprise value: $2.0B + $0.5B − $0.1B = $2.4 billion
3. Multiple: 2,400 / 300 = 8.0x

Say each step out loud as you go. Interviewers want to follow your logic.

Putting It Together: The Paper LBO From the Intro

Let's solve the question from the top of this post, with one added detail: the deal uses 60% debt, and $300 million of it gets paid down over five years.

Entry:

• EBITDA $100M × 10x = $1,000M purchase price
• Debt: 60% × 1,000 = $600M
• Equity: $400M

Exit:

• EBITDA $150M × 10x = $1,500M
• Remaining debt: 600 − 300 = $300M
• Equity value: 1,500 − 300 = $1,200M

Returns:

• MoM: 1,200 / 400 = 3.0x
• Over five years, 3.0x is roughly 25% IRR

Every step there used a skill from the list above. That's the point of drilling: when the pieces are automatic, you can focus on the logic.

How to Approximate Like a Banker

Exact answers are nice. Fast, reasonable answers are better.

• Round early, adjust later. 487 × 21 is close to 500 × 20 = 10,000. Actual answer: 10,227. Close enough to sanity check anything.
• Say "roughly" with confidence. "That's roughly 8x" sounds sharp. Silence for 40 seconds does not.
• Know which way you rounded. If you rounded both numbers up, your answer is too high. Mention it.
• Check against intuition. If you get a 90% EBITDA margin for a retailer, something went wrong.

A Simple Daily Mental Math Routine

Ten to fifteen minutes a day beats a two-hour cram session the night before.

Week 1: Build the Foundation

• Memorize the fractions table and compounding anchors
• Do 20 percentage problems a day (make up random numbers)
• Time yourself and track it

Week 2: Add Speed

• Practice multiplication shortcuts with two-digit numbers
• Drill the MoM-to-IRR table until you can recite it
• Calculate margins and growth rates from real company headlines you read

Week 3: Chain It Together

• Do one full paper LBO per day, out loud
• Solve EV and multiple questions with three or more steps
• Have a friend fire questions at you so you practice under mild pressure

Ongoing Habits

• Do mental math in daily life. Tips, discounts, splitting bills. Every bit counts.
• Talk through problems out loud. Interviews are verbal, so practice should be too.
• Review your misses. If you keep stumbling on division by 7, drill it specifically.

Common Mistakes to Avoid

• Losing track of units. Mixing millions and billions is the fastest way to tank a good answer.
• Going silent. Narrate your steps, even if you're unsure.
• Chasing precision you don't need. "About 25%" is a great answer to a paper LBO. "24.57%" is fine too, but don't spend a minute getting there.
• Skipping the sanity check. Always ask yourself, "Does this number make sense?"

The Bottom Line

Mental math in IB interviews isn't about being a human calculator. It's about having a small toolkit of shortcuts so automatic that your brain is free to think about the actual finance.

Memorize the fractions, the compounding anchors, and the IRR table. Drill percentages and multiplication daily. Then practice full problems out loud until they feel routine.

Start today with ten minutes. By the time that MD slides the blank sheet across the table, you'll be ready to smile and start writing.