The average sits midway between the two masses, so the total is twice the average. Take Boy 1 out to leave Boy 2.<br><br>[[model:questions/solutions/q4668.png]]<br><br>• Total → 2 × 51 = 102 kg<br>• Boy 2 → 102 − 53.5 = 48.5 kg<br><br>The other boy is 48.5 kg.<br>Check: (53.5 + 48.5) ÷ 2 = 102 ÷ 2 = 51 kg. ✓