The average sits midway between the two masses, so the total is twice the average. Take Boy 1 out to leave Boy 2.

Boy 153.5 kgBoy 2?average 51 kg

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

  1. Total → 2 × 51 = 102 kg
  2. Boy 2 → 102 − 53.5 = 48.5 kg

The other boy is 48.5 kg.

Check: (53.5 + 48.5) ÷ 2 = 102 ÷ 2 = 51 kg. ✓