Nicholas = 1 unit, father = 3 units. Together 4 units = 108 kg.

Nicholas1Father111108 kg

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

  1. 4 units → 108 kg
  2. 1 unit → 108 ÷ 4 = 27 kg

Nicholas is 27 kg.

Check: father 81 + Nicholas 27 = 108 kg, and 81 = 3 × 27. ✓