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# help Needed please

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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 two rolls is 5

May 2, 2020

#1
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The possible rolls are (1,4), (2,3), (3,2), (4,1), so the probability is 4/36 = 1/9.

May 2, 2020
#2
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Sorry it's incorrect...

LuckyDucky  May 2, 2020
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The sum could be either a 3, a 5, a 7, or an 11.

3  =  (1,2) or (2,1)

5  =  (1,4), (4,1), (2,3) or (3,2)

7  =  (1,6), (6,1), (2,5), (5,2), (3,4), or (4,3)

11  =  (5,6) or (6,5)

There are 14 winning possibilities out of a possible 36 possibilities:  14/36  =  7/18

May 2, 2020
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Sorry, its still wrong...

LuckyDucky  May 2, 2020
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Hi LuckyDucky!

I'm probably wrong too, but I'll give this problem a shot too :)

So I basically did the same thing as the Guest who answered your question, but I think the answer is actually $$\boxed{\frac{4}{15}}$$because the questions says "you are told that the sum of the two rolls is a prime"  the answer is actually 4 out of the prime sums, and there are 15 possible prime sums.  (The Guest made the mistake of making it out of all 36 possible sums)

So, I think the answer is $$\boxed{\frac{4}{15}}$$

I'm probably wrong, but I hope this helped you anyway

:)

May 2, 2020
edited by lokiisnotdead  May 2, 2020
#6
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Sorry its wrong..

LuckyDucky  May 2, 2020
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oops I just edited it, is it right now?

lokiisnotdead  May 2, 2020
edited by lokiisnotdead  May 2, 2020
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LuckyDucky  May 2, 2020
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oh wow, yayyyyy! you're welcome LuckyDucky :)

lokiisnotdead  May 2, 2020
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I really appreciate it by the way!

LuckyDucky  May 2, 2020
#8
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1+1 =2 .......1 permutations
2+1=3..........2p
2+3=5..........2p
4+1 =5..........2p
6+1=7...........2p
5+2=7............2p
3+4=7............2p
5+6=11..........2p
Total = 15 permutations
Probability = 15 / 6^2 =5 / 12

May 2, 2020