If somebody rolls 2 standard six-sided dice what is the probability he obtains a sum of 7 or 11?

Answer 1

2 out of 12

Ans:

1 2 3 4 5 6 1 2 3 4 5 6

total no N of sums of two numbers assuming dice are indistinguishable:

N = 36

ways to get 7:

1 and 6 with a chance of (1/6)*(1/6) = 1/36

2 and 5 wih a chance of 1/36

4 and 3 with a chance of 1/36

whats the chance of getting 7 with either of these combinations:

1/36 + 1/36+ 1/36 = 1/12

what about 11:

6 and 5, which is the only way to get an 11:

the prob of this is (1/6)*(1/6) = 1/36

the prob of getting a 7 or an 11, with two dice:

1/36 + 1/12 = 1/9!

Hopefully someone will jump in if I've made a mistake.

OK, I've been told the answer is 2/9. If you do the same working as above, but you'll have to assume there are two ways to get each sum:

example, if you get a 7 from 2 and 5, then you can also get 7 from a 5 and 2. This means the dice are distinguishable!