The three boxes together weigh 3 × the average. Take out the known total of X and Z to leave Y.<br><br>[[model:questions/solutions/q4677.png]]<br><br>• Total of 3 boxes → 3 × 810 = 2430 g<br>• X + Z → 2 kg 70 g = 2070 g<br>• Y → 2430 − 2070 = 360 g<br><br>Box Y weighs 360 g.<br>Check: (2070 + 360) ÷ 3 = 2430 ÷ 3 = 810 g. ✓