Suppose that two fair dice have been tossed and the total of their top faces is found to be divisible by 5. WHat is the probability that both of them have landed 5?
That the total is divisible by 5 means it is 5 or 10.
5 can be made by (r1,b4), (r2,b3), (r1,b4)
10 can be made (r4,b6), (r5,b5), (r6,b4)
These are all equally likely cases, so 1 in 6 cases where the total is divisible by 5 have both die showing 5, that is 1/6.