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PS - Good probablity question [#permalink]
17 Jun 2008, 09:32

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Tricky question, thought I'd share with the group. Took me 3 mins to solve

A fair, six-sided die, with sides numbered one through six, is rolled 5 times. What is the probability that on exactly 3 rolls the number of dots showing is greater than 4? A) 1/243 B) 10/243 C) 1/27 D) 40/243 E) 80/243

edit: I mistyped, it should say "What is the probability that on exactly..."

Last edited by xALIx on 17 Jun 2008, 15:02, edited 2 times in total.

Re: PS - Good probablity question [#permalink]
17 Jun 2008, 10:09

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I think answer is (D)

In order to have more than 4 on exactly 3 rolls, you have to get 5 or 6 on 3 rolls (chance this happens for 1 roll: 2/6) and 1,2,3 or 4 on the 2 others (chance this happens for 1 roll: 4/6).

Once you chose which of the 3 rolls will bear the 5 or 6, you have (2/6)^3 * (4/6)^2 chances it happens

This is 4/243

And there are 10 choices of the 3 rolls on 5 (unordered choices).

Re: PS - Good probablity question [#permalink]
17 Jun 2008, 15:04

x-ALI-x wrote:

Tricky question, thought I'd share with the group. Took me 3 mins to solve

A fair, six-sided die, with sides numbered one through six, is rolled 5 times. What is the probability that on exactly 3 rolls the number of dots showing is greater than 4? A) 1/243 B) 10/243 C) 1/27 D) 40/243 E) 80/243

edit: I mistyped, it should say "What is the probability that on exactly..."

For this question, the answer is 40/243, the key is including the combinations so 4/243*10

Re: PS - Good probablity question [#permalink]
17 Jun 2008, 15:05

I'll state the question differently, for similar practice:

A fair, six-sided die, with sides numbered one through six, is rolled 5 times. What is the probability that on exactly 3 rolls the number of dots showing is no greater than 4? A) 1/243 B) 10/243 C) 1/27 D) 40/243 E) 80/243

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