In a class of 50 students, there are 27 boys whose average weight is 72 kg, and the average weight of the whole class is 55.44 kg. What is the average weight in kilograms of the girls in the class?

Introduction / Context:
This quantitative aptitude problem tests your understanding of weighted averages and how to separate the average of a subgroup (girls) from information about the whole group (entire class) and another subgroup (boys). Such questions are very common in competitive exams because they combine basic arithmetic with logical reasoning about groups and totals.

Given Data / Assumptions:

Total number of students in the class = 50.
Number of boys = 27 and therefore number of girls = 50 - 27 = 23.
Average weight of the boys = 72 kg.
Average weight of the full class = 55.44 kg.
Weights are measured in kilograms and we assume all given averages are correct and precise.

Concept / Approach:
The key idea is that average = total weight / number of students. From the overall average and the class size, we can find the total weight of all students. From the boys' average and the number of boys, we can find the total weight of the boys. Subtracting the boys' total from the class total gives the total weight of the girls. Dividing that by the number of girls gives the girls' average weight.

Step-by-Step Solution:
Step 1: Compute total weight of all 50 students. Total class weight = 55.44 * 50 = 2772 kg. Step 2: Compute total weight of the 27 boys. Total boys' weight = 72 * 27 = 1944 kg. Step 3: Find total weight of the 23 girls. Total girls' weight = 2772 - 1944 = 828 kg. Step 4: Compute average weight of the girls. Average girls' weight = 828 / 23 = 36 kg.

Verification / Alternative Check:
We can verify by recomputing the overall average. Boys contribute 1944 kg and girls contribute 828 kg, giving a total of 2772 kg. Dividing by 50 students gives 2772 / 50 = 55.44 kg, which matches the given class average. This confirms that the girls' average weight of 36 kg is consistent with all the data.

Why Other Options Are Wrong:
42 kg and 48 kg are too high; if girls were that heavy on average, the class average would be much closer to the boys' average of 72 kg. That would contradict the relatively low class average of 55.44 kg.
30 kg is too low; it would pull the class average significantly below 55.44 kg once combined with the heavier boys, which is not correct.

Common Pitfalls:
A common mistake is to simply average 72 kg and 55.44 kg or to guess a value between them, which ignores the different group sizes. Another frequent error is to forget to compute totals and instead directly try to manipulate averages. Always remember that when subgroups are involved, you must work through total sums before returning to averages.

Final Answer:
The average weight of the girls in the class is 36 kg.