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What this permutation-counting interview question tests

This is an easy combinatorics problem that quant interviewers use to check whether a candidate can quickly count arrangements of objects when some elements are identical. It is a foundational skill for probability reasoning and appears frequently in trading and research interviews.

To solve problems of this type, you identify the total number of positions, count how many times each distinct element appears, and apply the formula for permutations with repetition. The key is careful enumeration and avoiding the common mistake of treating identical items as distinct.

• Factorials and their cancellation in division
• Accounting for repeated elements
• The permutation formula for multisets