Math Is Fun Forum

sterling94

Member

Registered: 2018-09-28

Posts: 1

Question

Hi, two dice are thrown, find the probability that the sum is 8 if you know one of them is 5.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,640

Re: Question

Hi Sterling94,

Welcome to the forum.

When two dice are thrown there are 36 possible outcomes but we can limit ourselves to just (5,x) 6 possibilities plus (x,5) another 6 possibilities less the one we've counted twice (5,5).  So 11 in total.  Of these two give a total of 8 so 2/11

Bob

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Children are not defined by school ...........The Fonz
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Grantingriver

Member

Registered: 2016-02-01

Posts: 129

Re: Question

I think this question is a little bit tricky. The answer which is provided is correct if the final phrase was “if one of them must be five”. However, the phrase is stating that “if you know one of them is five”, so how could you know that a number is exsit without its actual existence? Therefore, I think it is more appropriate in the later case to have one of the dice has the number five already exist on its upper face and the other has a possible outcome of six values hence the answer in this case would be 1/6.