What combinatorics do I need for quant interviews?

Permutations and combinations, the multiplication and addition principles, inclusion–exclusion, and stars-and-bars cover the vast majority of what's asked. They almost always appear in service of a probability question.

How can I check a combinatorics answer quickly?

Test it on a tiny version of the problem you can count by hand, and watch for double-counting. Framing the count as 'choose, then arrange' consistently avoids the most common errors.

Are combinatorics and probability questions separate?

Rarely - counting is usually the first step of a probability problem. Strong counting makes the probability fall out, which is why interviewers test it heavily.

Combinatorics Interview Questions

Counting cleanly is a quant superpower. These combinatorics questions cover permutations, combinations, and inclusion–exclusion that show up across probability problems.

This area covers permutations and combinations, the multiplication and addition principles, inclusion–exclusion, and stars-and-bars - the tools behind a large share of probability problems.

Most combinatorics mistakes are double-counting or miscounting the sample space. Repeated practice trains you to choose the right counting argument and to verify it on small cases.

196 combinatorics questions · 152 free to practise now.

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Frequently asked questions

What combinatorics do I need for quant interviews?: Permutations and combinations, the multiplication and addition principles, inclusion–exclusion, and stars-and-bars cover the vast majority of what's asked. They almost always appear in service of a probability question.
How can I check a combinatorics answer quickly?: Test it on a tiny version of the problem you can count by hand, and watch for double-counting. Framing the count as 'choose, then arrange' consistently avoids the most common errors.
Are combinatorics and probability questions separate?: Rarely - counting is usually the first step of a probability problem. Strong counting makes the probability fall out, which is why interviewers test it heavily.