There are five boxes in a truck. The weight of the first box is 300 kg and the weight of the second box is 20% higher than the weight of the third box, whose weight is 25% higher than the first box’s weight. The fourth box at 400 kg is 20% lighter than the fifth box. Find the difference in the average weight of the four heaviest boxes and the four lightest boxes.

Box 1 = 300 kg. Box 3 = 300 × 1.25 = 375 kg. Box 2 = 375 × 1.2 = 450 kg. Box 4 = 400 kg, which is 20% lighter than Box 5, so 400 = 0.8 × Box 5, giving Box 5 = 500 kg. Sorted: 300, 375, 400, 450, 500. Average of 4 lightest = (300+375+400+450)/4 = 381.25 kg. Average of 4 heaviest = (375+400+450+500)/4 = 431.25 kg. Difference = 50 kg. Option A gives 60. Option C gives 80. Option D gives 90.