Both rows contain the same 450 g bag, so the bag cancels out. The extra mass in the first row comes only from the 7 − 4 = 3 extra boxes.<br><br>[[model:questions/solutions/q8366.png]]<br><br>• Difference → 3950 − 2450 = 1500 g<br>• 3 boxes → 1500 g<br>• 1 box → 1500 ÷ 3 = 500 g<br><br>One box has a mass of 500 g.<br>Check: 7(500) + 450 = 3950 g; 4(500) + 450 = 2450 g. ✓