a is chosen from the set {1,2,3,4,5}, and b is chosen from the set {5,6,7,8,9}. What is the probability that the product ab is a prime number?
5 6 7 8 9
1 5 6 7 8 9
2 10 12 14 16 18
3 15 18 21 24 27
4 20 24 28 32 36
5 25 30 35 40 45
We have 25 outcomes.....only 2 are prime
So
2 / 25