Take 5C as 1 unit. 5B is 2 more than 5C, and 5A is 3 more than 5C. The three classes make 3 units + 5 = 119.

5C15B125A13119

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

  1. 5A + 5B + 5C = 3 units + 2 + 3 = 3 units + 5 → 119
  2. 3 units → 119 − 5 = 114
  3. 1 unit → 114 ÷ 3 = 38 (5C)
  4. 5B + 5C = (1 unit + 2) + 1 unit = 2 units + 2 = 76 + 2 = 78

5B and 5C have 78 pupils altogether.

Check: 5C 38 + 5B 40 + 5A 41 = 119. ✓