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.<br><br>[[model:questions/solutions/q4691.png]]<br><br>• Total → 2 × 42 = 84 kg<br>• 4 units → 84 kg<br>• 1 unit (lighter) → 84 ÷ 4 = 21 kg<br>• Heavier → 3 × 21 = 63 kg<br><br>The heavier boy has a mass of 63 kg.<br>Check: (63 + 21) ÷ 2 = 42 kg and 63 = 3 × 21. ✓