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Wu rolls two fair, six-sided dice.  You are not told what the rolls were, but you are told that the sum of the two rolls is a prime.  What is the probability that the sum of the rolls is 3?

 Dec 13, 2021
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On a roll of 2 dice, there are 15 ways of getting a prime number. But, there are only 2 ways of getting the sum of 3, and that is: 1+ 2 and 2+1: 

 

Therefore, the probability is: 2 / 15

 

(1, 1)(1, 2) ,  (1, 3) ,  (1, 4) ,  (1, 5) ,  (1, 6)(2, 1) ,  (2, 2) ,  (2, 3) ,  (2, 4) ,  (2, 5) ,  (2, 6) ,  (3, 1) ,  (3, 2) ,  (3, 3) ,  (3, 4) ,  (3, 5) ,  (3, 6) ,  (4, 1) ,  (4, 2) ,  (4, 3) ,  (4, 4) ,  (4, 5) ,  (4, 6) ,  (5, 1) ,  (5, 2) ,  (5, 3) ,  (5, 4) ,  (5, 5) ,  (5, 6)(6, 1) ,  (6, 2) ,  (6, 3) ,  (6, 4) ,  (6, 5) ,  (6, 6) ,  Total == 36

 Dec 13, 2021

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