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# help

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Mahowny rolls two fair, six-sided dice. You don't know 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 two rolls is 7?

Dec 1, 2019

#1
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Mahowny rolls two fair, six-sided dice. You don't know 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 two rolls is 7?

prime number rolls

1,1   added after guest pointed out my error of omision    1

1,2    2,1                                              2

1,4    4,1    23,  32,                              4

1,6     2,5    3,4   doubled                     6

5,6   6,5                                               2

Total                                                   15     edited

probability is     6/15

Thanks for picking up my error guest :)

Dec 1, 2019
edited by Melody  Dec 1, 2019
#2
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hi melody

i think the answer is 2/5 (simplify from 6/15)

because 1,1 is a combination too, 2 is a prime

Guest Dec 1, 2019
#3
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Yes you are right :)

Melody  Dec 1, 2019
#4
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The sample space is 6^2 =36

The probability of rolling a prime number is 15 / 6^2 =5 / 12

The probability of a rolling a sum of 7 is 6 / 36 = 1 / 6

Dec 1, 2019
#5
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You have used the wrong sample space.

Melody  Dec 1, 2019