A number is picked at random from 1 to 20. What is the probability that it is a prime number?

Explanation

1. 01

   The primes up to 20 are 2, 3, 5, 7, 11, 13, 17 and 19: 8 numbers.

2. 02

   The probability is $\frac{8}{20} = \frac{2}{5}$.