Nicholas = 1 unit, father = 3 units. Together 4 units = 108 kg.<br><br>[[model:questions/solutions/q2521.png]]<br><br>• 4 units → 108 kg<br>• 1 unit → 108 ÷ 4 = 27 kg<br><br>Nicholas is 27 kg.<br>Check: father 81 + Nicholas 27 = 108 kg, and 81 = 3 × 27. ✓