Question 250304: Two dice are rolled. What is the probability of rolling the sum of 7? What about 7 OR 11?
Found 2 solutions by Greenfinch, drk:
Answer by Greenfinch(383)   (Show Source):
You can put this solution on YOUR website! To do this use a Sample Space diagram. With it the problem is ridiculously simple, without it it is not at all easy. Number is the addition of the values
Value of first die across top, value of second die down the side
1 2 3 4 5 6
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1 | 2 3 4 5 6 7
2 | 3 4 5 6 7 8
3 | 4 5 6 7 8 9
4 | 5 6 7 8 9 10
5 | 6 7 8 9 10 11
6 | 7 8 9 10 11 12
There are 6 x 6 or 36 options, all are equally likely, 7 occurs 6 times, so the chances are 6/36 or 1/6. 11 occurs 2 times so chances are 2/36 or 1/18. 7 or 11 are 8 of the 36 options so 8/36 or 2/9. You can even find out chance of a prime number. the 2,3,5,7,11 options add up to 15, so 15/36 or 5/12 is the chance of that.
Answer by drk(1908)  (Show Source):
You can put this solution on YOUR website! There are 6 ways to get a 7 and a total of 36 ways for 2 dice, so
P(7) = 6/36 = 1/6
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There are 2 ways to get a sum of 11. So,
P(7 OR 11) = 6/36 + 2/36 = 8/36 = 2/9.
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OR means add (+)
AND means multiply (*)
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