Matching pair from two red and two black socks

Question

A drawer contains exactly four socks: two red and two black. You pull out two at random. How likely are they to be the same colour?

Short answer

$1/3$ . Whatever the first sock is, only one of the remaining three socks shares its colour.

Adapted from Joshi, Denson & Downes, Quant Job Interview Questions and Answers, Q3.20, p.82; question and answer rewritten by SelfMadeQuant.com.

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Lessons behind this question

Counting and Combinatorics
Sample Spaces, Events, and the Probability Axioms
Conditional Probability, Total Probability, and Bayes' Rule

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