Weighted class average: A class has 15 boys and 10 girls with an overall average weight of 38.4 kg. If the boys average 40 kg, find the average weight of the girls.

Introduction / Context:
The task is a direct application of weighted averages. Knowing the overall mean and one subgroup mean, we can determine the other subgroup mean by working with totals (mean * count).

Given Data / Assumptions:

• Boys = 15, girls = 10
• Overall average = 38.4 kg
• Boys average = 40 kg

Concept / Approach:
Total weight of all = overall average * total students. Total of boys = boys average * number of boys. Subtract to get girls’ total, then divide by number of girls for girls’ mean.

Step-by-Step Solution:

Verification / Alternative check:
Check weighted recombination: (600 + 360) / 25 = 960 / 25 = 38.4 kg, matching the given overall average.

Why Other Options Are Wrong:

• 36.5, 35.6, 35, 37 kg: do not satisfy the weighted total when recomputed.

Common Pitfalls:
Taking an unweighted mean of 40 and an assumed girls mean without weighting by group sizes.

Final Answer:
36 kg