The three numbers add up to 3 × their average. Take out the two numbers (2 × their average) to leave the third.

2 numbers = 30?total = 3 × 16 = 48

Plain numbers are given; italic underlined numbers are worked out.

  1. Total of 3 → 3 × 16 = 48
  2. Total of 2 → 2 × 15 = 30
  3. Third number → 48 − 30 = 18

The third number is 18.

Check: (30 + 18) ÷ 3 = 48 ÷ 3 = 16. ✓