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 November 2nd, 2017, 07:11 PM #1 Newbie   Joined: Oct 2017 From: United States Posts: 7 Thanks: 1 Prob. Question This is somewhat confusing to me. [ Four people are selected at random. What is the probability that at least two of them were born on the same day of the week? ] Any methods to solve this? Do days of the week matter, perhaps? Thanks!
 November 2nd, 2017, 07:53 PM #2 Senior Member     Joined: Sep 2015 From: USA Posts: 2,124 Thanks: 1103 one has to assume that it's equally likely to be born on each day. That's probably pretty close to being true, but it's an assumption nonetheless. \begin{align*} &P[\text{2 or more born on same day of the week}] = \\ \\ &1 - P[\text{all born on different days of the week}] = \\ \\ &1 - \dfrac{7\cdot 6 \cdot 5 \cdot 4}{7^4} = \\ \\ &1 - \dfrac{120}{343} = \dfrac{223}{343} \approx 0.65 \end{align*} Thanks from Country Boy and Capitalism

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