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What is the probability of getting double dice? (one throw)

All possible dice rolls are 36 (counting even rolls like 1-4 and 4-1).

But when counting the doubles, why not count 1-1 twice, for example?

My book says the probability is 6/36.

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Something to consider here is if you had more than 2 throws, would you still call some things duplicates or not? For example, if you did 4 dice would 1-1-3-4 be considered or duplicate or would that be just 4-4-4-4? –  JB King Feb 7 '13 at 17:46
    
Or consider throwing the two dice one after the other. First one can land any way it wants. Second has 6 ways to land, only 1 will finish the double. Same answer, and it's reduced to lowest terms for free! –  User58220 Feb 7 '13 at 23:32

2 Answers 2

up vote 4 down vote accepted

You don't count (1,1) twice, because the situation "both dice show a 1" can only occur in one way - the first dice is 1, and the second dice is 1.

By contrast, the situation "one dice shows a 4, the other shows a 1" can occur in two ways - either the first dice is 4 and the second is 1, or the first dice is 1 and the second is 4.

The easiest way to convince yourself of this is to tabulate all 36 possibilities:

1 1    1 2    1 3    1 4    1 5    1 6
2 1    2 2    2 3    2 4    2 5    2 6
3 1    3 2    3 3    3 4    3 5    3 6
4 1    4 2    4 3    4 4    4 5    4 6
5 1    5 2    5 3    5 4    5 5    5 6
6 1    6 2    6 3    6 4    6 5    6 6
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Thank you very much! –  Ilea Cristian Feb 7 '13 at 17:40

Before you roll your dice, colur one of them red and the other blue. Then red $1$ and blue $1$ is the same thing as blue $1$ and red $1$.

But red $1$ and blue $5$ is not the same as blue $1$ and red $5$.

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