The two volumes add up to twice the average. The fuller bottle is 4 units; the other is 1 unit, so 5 units in all.

Less1More1111total = 2 × 625 = 1250 ml

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

  1. Total → 2 × 625 = 1250 ml
  2. 5 units → 1250 ml
  3. 1 unit (less) → 1250 ÷ 5 = 250 ml
  4. More → 4 × 250 = 1000 ml

The bottle with more water has 1000 ml and the bottle with less water has 250 ml.

Check: (1000 + 250) ÷ 2 = 625 ml and 1000 = 4 × 250. ✓