What is the probability that a list of five different, single-digit numbers, whose sum is 18, will contain exactly two prime numbers? Express your answer as a common fraction.
I think that there are only two possible lists:
1, 2, 3, 4, 8 and 1, 2, 4, 5, 6
Both of these lists have exactly two prime numbers; so the probability is 2/2.
