In a class of 39 students, there are 26 girls with an average weight of 42 kg. The average weight of the entire class is 48 kg. What is the average weight (in kg) of the boys in the class?

Introduction:
This is a weighted-average problem. Knowing the average for the whole class and for a subgroup (girls), we can compute the subgroup complement (boys) average by converting to totals and subtracting.

Given Data / Assumptions:
Total students = 39. Girls = 26 with average 42 kg. Boys = 39 − 26 = 13. Class average = 48 kg.

Concept / Approach:
Compute total class weight and total girls' weight. The difference is total boys' weight. Divide by number of boys to get boys' average weight.

Step-by-Step Solution:
Total class weight = 39 * 48 = 1872 kg. Total girls' weight = 26 * 42 = 1092 kg. Total boys' weight = 1872 − 1092 = 780 kg. Boys' average = 780 / 13 = 60 kg.

Verification / Alternative check:
Weighted average using 26 at 42 and 13 at 60 yields (26*42 + 13*60) / 39 = (1092 + 780) / 39 = 1872 / 39 = 48, consistent with the class average.

Why Other Options Are Wrong:
54, 58, 62, 66: None produce the correct overall average when combined with 26 girls at 42 kg.

Common Pitfalls:
Averaging 42 and 48 directly or dividing by the wrong headcount. Always work with totals for weighted-average questions.

Final Answer:
The average weight of the boys is 60 kg.