Explanation:
{0,1,2,3,4,5,6,7,8,9}--set of numbers from 0 to 9
{2,3,5,7}---set of prime numbers from 0 tp 9
there are 10 numbers in the first set and 4 numbers in the second set.
4/9 is the probability that the digit he selects is a prime number
Hope that helps :)