The average of five numbers is 20. The average of the first two is 15 and the average of the last two is 24. What is the middle (third) number?

Explanation

1. 01

   Total of five = $5 \times 20 = 100$.

2. 02

   First two total = $2 \times 15 = 30$; last two total = $2 \times 24 = 48$.

3. 03

   Third = $100 - 30 - 48 = 22$.