prime number is a whole number greater than 1 whose only factors are 1 and itself. A factor is a whole number that can be divided evenly into another number. The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19
In this problem there's 9 prime numbers out of 20 so our probably is 9/20 because you have nine chances of geting a prime out of 20 balls total.