The three boxes together weigh 3 × the average. Take out the known total of X and Z to leave Y.

X + Z = 2070 gY = ?total = 3 × 810 = 2430 g

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

  1. Total of 3 boxes → 3 × 810 = 2430 g
  2. X + Z → 2 kg 70 g = 2070 g
  3. Y → 2430 − 2070 = 360 g

Box Y weighs 360 g.

Check: (2070 + 360) ÷ 3 = 2430 ÷ 3 = 810 g. ✓