Investment banking brain teasers, worked out loud
Investment banking brain teasers are short and usually end in a number: a sum, a percentage, a present value or a probability. Morgan Stanley says it doesn't always expect the right answer and wants to see how you work through a scenario, so say each step out loud and finish with a quick check. Soreno, an AI voice interview practice app for consulting, investment banking and other finance candidates, runs investment banking interview practice where the interviewer hears your working as well as your answer.
Updated
- Mostly about money. Most banking teasers are about percentages, compounding and time value.
- No calculator. Square roots, products and fractions are done in your head, out loud.
- The working counts. Morgan Stanley says it wants to see how you work through a scenario.
- Check it twice. A second route to the same number is the habit bankers notice.
- Not every round. Mental math or a teaser shows up in 30 of 331 interview reports we read.
How banking brain teasers differ
Consulting puzzles test logic. Banking teasers mostly test whether you are quick and careful with numbers that look like the job.
What makes them banking teasers3 traits
The topic
Money in the question
A stock that falls and recovers, a sum that compounds, cash today against cash later. The same ideas sit under every DCF and returns question.
The pace
Fast, in your head
Many are mental math: a product, a square root, a fraction. Thirty seconds is a fair target for the short ones.
The classics
Logic puzzles are rarer
Monty Hall, the 12 coins and the river crossing come up less in banking. They are worked in our general guide.
Classic brain teasers, worked
One worked out loud: $100 now or $10 a week?Present value
Would you rather have $100 today or $10 a week for the next 10 weeks?
Step 1
Name the catch
Both pay $100 in total. So the question is about timing and risk.
Step 2
Take the $100 now
Money today can earn interest, so at any positive rate the weekly stream is worth less than $100. It also carries the risk that a payment never comes.
Step 3
Put a number on it
The average payment arrives after 5.5 weeks. At 5% a year, that costs about 5% × 5.5/52, roughly half a percent. So the stream is worth about $99.47.
Step 4
Check it the long way
Discount each of the 10 payments at 5%/52 a week and add them up: $99.47 again.
Follow-up
What weekly payment makes it even?
About $10.05: $10 × 100 ÷ 99.47. Any more and the stream wins, if you trust the payer.
What the interviewer listens for
No bank publishes a brain teaser rubric. Morgan Stanley says it doesn't always expect the right answer and wants to see how you work through a scenario.
The setup, before the math
Repeat the question and say your approach in one line: “I'll pair the numbers” or “I'll work out the odds of none and subtract.”
Each step, spoken
A right answer after twenty silent seconds tells them less than a slower answer they could follow.
A second route
Check 5,050 as the average, 50.5, times 100 numbers. Bankers check every model this way.
Calm when it goes wrong
If you catch a slip, say so and fix it. Morgan Stanley also asks candidates to take their time and communicate concisely.
12 investment banking brain teasers, worked
Each was reported by candidates in a banking interview. Open one for the working and the follow-ups that tend to come next.
Mental math4 teasers
No paper, no calculator. Say the shortcut you are using.
What is the sum of the numbers from 1 to 100?
Show the working
5,050. Pair 1 with 100, 2 with 99 and so on: 50 pairs of 101, and 50 × 101 = 5,050.
- “And from 1 to 1,000?”
- “Only the odd numbers from 1 to 99?”
Which is bigger, 51 × 49 or 50 × 50?
Show the working
50 × 50, by exactly 1. 51 × 49 is (50 + 1)(50 − 1) = 2,500 − 1 = 2,499.
- “What is 48 × 52?”
- “What is 45 squared?”
Roughly, what is the square root of 341?
Show the working
About 18.5. 18² is 324 and 19² is 361, and 341 sits 17 above 324. Adding 17 ÷ 36 to 18 gives about 18.47.
- “Why divide by 36?”
- “How would you check 18.47 quickly?”
How many zeros are at the end of 100 factorial?
Show the working
24. Each zero needs a 2 and a 5, and 5s run out first. There are 20 multiples of 5 up to 100, plus 4 more 5s from 25, 50, 75 and 100.
- “And 1,000 factorial?”
- “Why not count the 2s?”
Percentages and money4 teasers
The ones closest to the job. Each has a trap answer that sounds right.
A stock falls 10%. How much must it rise to get back to where it started?
Show the working
About 11.1%, not 10%. From $100 it falls to $90, and $10 is 1/9 of $90. The rise needed is the loss divided by what is left.
- “And after a 20% fall?”
- “Why does a 50% fall need a 100% gain?”
Your investment is up 20% and is now worth $60. What did you put in?
Show the working
$50, because $60 is 1.2 times the start and 60 ÷ 1.2 = 50. Taking 20% off $60 gives $48, which is the trap.
- “What if it were down 20% to $60?”
What is $100 worth after three years at 10% a year?
Show the working
$133.10: $110, then $121, then $133.10. The $3.10 above simple interest is interest earned on interest.
- “What is $133.10 in three years worth today?”
- “And at 10% for five years?”
Would you rather have $100 today or $10 a week for 10 weeks?
Show the working
The $100 today. Both total $100, but the stream arrives later and might not arrive at all. At 5% a year it is worth about $99.47.
- “What weekly payment makes them even?”
- “Does your answer change if rates are zero?”
Odds2 teasers
Short probability. Counting the opposite case is often faster.
You flip a coin three times. What is the chance of at least one head?
Show the working
7 in 8, or 87.5%. The only way to miss is three tails, a 1 in 8 chance, so take 1 minus 1/8.
- “Exactly two heads?”
- “At least one six in four rolls of a die?”
A drawer has 10 blue, 15 red and 20 yellow socks. How many must you pull, without looking, to be sure of a matching pair?
Show the working
Four. With three colors, three socks can all differ, and the fourth must match one of them. The counts of each color don't matter.
- “To be sure of a blue pair?”
Clocks and estimates2 teasers
One exact answer and one estimate. State assumptions on the estimate.
What is the angle between a clock's hands at 3:15?
Show the working
7.5 degrees, not zero. The minute hand is on the 3, but the hour hand has moved a quarter of the 30 degrees toward the 4.
- “At 9:45?”
- “At 12:30?”
How many golf balls fit in a Boeing 747?
Show the working
About 20 million: a tube about 20 feet across and 200 feet long, less seats and galleys, holding roughly 400 loose balls per cubic foot. Give a range at the end. Practice it, see a model answer
- “What would change your answer most?”
- “How would you get a better number?”
More to prepare with
Where candidates slip
Answering with the reflex number
10% back after a 10% fall, $48 for the $60 question, zero degrees at 3:15. Pause long enough to test the obvious answer.
Going quiet to calculate
Say the method first, then the arithmetic. Silence reads as stuck even when you aren't.
Refusing to estimate
“I'd need to look it up” ends the question. Make an assumption, say it, and move on.
Rounding too early
On a chain of steps, keep one extra digit until the end, then round the answer you give.
Ten minutes a day before your interview
Short daily reps beat one long session the night before.
Squares
Squares to 25 by heart
They make square roots and products like 51 × 49 instant.
Fractions
Eighths, ninths and twelfths as percentages
1/8 is 12.5%, 1/9 is 11.1%, 1/12 is 8.3%. These turn up in every percentage teaser.
Growth
Powers of 1.05, 1.1 and 1.2
1.1 cubed is 1.331, 1.2 squared is 1.44. You will use them in returns questions too.
Out loud
Say one teaser a day to someone
Or record yourself. Listen for the setup line and the check at the end.
Thirty seconds, a number, and someone listening
Soreno puts quick math between real banking technicals, the way an interviewer does, and asks how you got there. Start your free week.
Questions people ask
Do investment banks still ask brain teasers?
Some interviewers do, but not in every round. Candidates mention mental math or a brain teaser in 30 of the 331 banking interview reports on Wall Street Oasis that we read in October 2026. Technical and fit questions take most of the time.
Which banks ask brain teasers?
No bank publishes its questions. Treat them as possible anywhere and keep a few minutes of mental math sharp.
What kinds of brain teasers do banks ask?
Mostly mental math and money: products and square roots without a calculator, a stock that falls and has to recover, compounding and present value. Short odds questions and the odd estimate, like golf balls in a plane, come up too.
Where can I practice brain teasers for free?
Our brain teaser trainer gives you 10 timed puzzles a day with the answer and the reasoning after each one, no sign-up. For the banking ones, say the 12 on this page out loud, then try a full mock interview where a teaser comes between technical questions.
What if I can't solve it?
Keep talking. Say what you know, what you would try next and why. Morgan Stanley says it doesn't always expect candidates to have the right answer.
How are banking brain teasers different from consulting ones?
Banking ones are shorter and usually about money: percentages, compounding and present value. Consulting interviews lean on market sizing and logic. Our general brain teaser guide covers the classic logic puzzles.
Can I use paper for a brain teaser?
Ask. In person, a notepad is often fine for an estimate. On a call or a video screen, assume mental math and say each step so the interviewer can follow.