Suppose that you roll two dice and don't get to look at the outcome. your friend looks at the outcome and tells you honestly that at least one of the dice came up 6. what is the probability that the sum of your two dice is 8?

The possible cases that at least one 6 is shown are:

(1,6), (2,6), (3,6), (4,6), (5,6), (6,6), (6,5), (6,4), (6,3), (6,2), (6,1)

The possible cases that at least one 6 is shown and their sum is 8: (6,2), (2,6)

Hence the probability is 2/11