The two masses add up to twice the average. The heavier boy is 3 units and the lighter is 1 unit, so 4 units in all.

Lighter1Heavier111total = 2 × 42 = 84 kg

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

  1. Total → 2 × 42 = 84 kg
  2. 4 units → 84 kg
  3. 1 unit (lighter) → 84 ÷ 4 = 21 kg
  4. Heavier → 3 × 21 = 63 kg

The heavier boy has a mass of 63 kg.

Check: (63 + 21) ÷ 2 = 42 kg and 63 = 3 × 21. ✓