Explanation

The class average must be a weighted average because the two groups are of different sizes — you cannot simply average 55 and 58. The 20 girls contribute 20 × 55 = 1100 marks in total, and the 30 boys contribute 30 × 58 = 1740 marks, for a class total of 2840 marks across 50 students. Dividing, 2840/50 = 56.8. Option A (56.5), the plain average of the two averages, would only be correct if the groups were equal in size; since the higher-scoring boys’ group is larger, the true average is pulled above 56.5 toward 58. Hence the class average is 56.8, option D.