Combinatorics Quant Interview Questions
How to handle quant interview counting questions: permutations, combinations, inclusion-exclusion, symmetry, and complements.
Candidates who lose points through overcounting or undercounting.
What counting questions test
Counting questions test whether you can define distinct outcomes. Most errors come from using permutations when combinations are needed, or from forgetting overlap when multiple conditions can happen together.
Concrete example
The number of ways to choose three people from ten is C(10,3), not 10 x 9 x 8, unless the order of selection matters. If a probability problem says "hand," "set," or "committee," order usually does not matter.
How to practice
Ask three questions before calculating: are outcomes ordered, can items repeat, and are there overlapping cases? Then choose combinations, permutations, stars and bars, complement counting, or inclusion-exclusion.
Common mistakes
Candidates often count the same object multiple ways and then divide by the wrong symmetry factor. If you divide by something, be able to say exactly what duplicate labels you are removing.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.