In a class of 55 students, 34 are girls with an average weight of 51 kg and the average weight of the whole class is 55.2 kg. What is the average weight of the boys in the class (in kg)?

Introduction / Context:
This problem is a classic example of a weighted average question in aptitude tests. We know the overall average weight of the entire class, as well as the number and average weight of one subgroup, namely the girls. The task is to use this information to compute the average weight of the remaining subgroup, the boys. Such questions test understanding of how averages relate to totals and how to separate contributions of different groups.

Given Data / Assumptions:
- Total number of students in the class = 55
- Number of girls = 34
- Average weight of girls = 51 kg
- Average weight of the whole class = 55.2 kg
- Therefore number of boys = 55 minus 34 = 21 boys
- We assume all weights are measured accurately and in kilograms

Concept / Approach:
The key idea is that total weight equals average weight multiplied by the number of students. First we compute the total weight of the entire class using the overall average. Then we compute the total weight of all girls using their average. Subtracting the girls total from the class total gives the total weight of the boys. Finally we divide that boys total by the number of boys to obtain the required average weight.

Step-by-Step Solution:
Step 1: Compute total weight of the class: total class weight = 55.2 × 55 = 3036 kg.Step 2: Compute total weight of the girls: girls total weight = 34 × 51 = 1734 kg.Step 3: Compute total weight of the boys: boys total weight = 3036 − 1734 = 1302 kg.Step 4: Number of boys = 21, so boys average weight = 1302 ÷ 21 = 62 kg.Step 5: Therefore the required average weight of the boys is 62 kg.

Verification / Alternative check:
If boys average weight is 62 kg, total boys weight is 62 × 21 = 1302 kg. Adding girls total 1734 kg gives 1302 + 1734 = 3036 kg, which matches the earlier computed total class weight. Dividing 3036 by 55 again gives 55.2 kg, confirming that our result is consistent with the original overall average, so the calculation is correct.

Why Other Options Are Wrong:
Values like 59.4 kg or 56.8 kg would lead to a boys total weight that does not sum to 3036 kg when combined with the girls total. Similarly, 60 kg or 64 kg give combined totals that do not reproduce the class average of 55.2 kg, so these alternatives are mathematically inconsistent with the given data.

Common Pitfalls:
A common mistake is to average 51 kg and 55.2 kg directly, which is invalid because the two groups have different sizes. Another frequent error is to divide the difference in total weight by the total number of students rather than by the number of boys. Forgetting to compute the correct number of boys from the difference between total students and girls also leads to wrong answers.

Final Answer:
The average weight of the boys in the class is 62 kilograms.