We roll two fair dice. what is the probability that at least one die shows a three, given that the sum of the dice is 4?

You have three choices if the sum of the dice is four. 1- the first die is 3, and the second is 1. 2 - the first die is 1, and the second is 3. 3 - Both die are 2. Therefore, 2/3 of your choices give you one die that equals three, and that gives you your probability.

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