A = 6 units, C = 2 units. A has 84 ml more than C, which is the 4-unit difference.

ABCA − C = 4 units = 84 ml

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

  1. A − C → 6 units − 2 units = 4 units → 84 ml
  2. 1 unit → 84 ÷ 4 = 21 ml
  3. B → 7 units = 7 × 21 = 147 ml

Container B has 147 ml of water.

Check: A 126, C 42; 126 − 42 = 84; B = 7 × 21 = 147. ✓