Posted by **Lori** on Sunday, September 19, 2010 at 9:37pm.

Roll two standard dice and add the numbers. What is the probability of getting a number larger that 9 for the first time on the third time.

- Statistics -
**bobpursley**, Sunday, September 19, 2010 at 9:43pm
larger than 9? that means 10,11, 12

Well, to get a 12, you can do it one way: Pr=1/6*1/6

To get an 11, you can do it two ways (5,6, or6,5) Pr= 2*1/6*1/6

To get a 10, you can do it 64,46,5,5 three ways

Pr=3*1/6*1/6

add the probabilities of all the combinations.

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**bobpursley**, Sunday, September 19, 2010 at 9:45pm
Actually, rereading the question, I dont know what "for the first tme on the third time" means.

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**Lori**, Sunday, September 19, 2010 at 9:52pm
I seem to get 0.167--book has 0.1157

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**Lori**, Sunday, September 19, 2010 at 10:24pm
Thank you for trying--appreciate it.

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**PsyDAG**, Monday, September 20, 2010 at 11:31am
It means that the first two times you get a 9 or less, but the third time you get a 10, 11 or 12.

For third time, add the probabilities from Bobpursley. For either-or probabilities, add individual probabilities.

(1/6*1/6) + (2*1/6*1/6) + (3*1/6*1/6) = ?

Probability of 9 or less = 1 -?

For probability of all events, multiply probabilities of individual events. Thus the answer to your problem is:

(1-?)(1-?)(?) = ??

I'll let you do the calculations.

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