In a class, there are 60 students. The average weight of the entire class is 42 kg. If there are 36 boys in the class and the average weight of all the boys is 50 kg, then what is the average weight (in kg) of all the girls?

The correct option is 30.

To find the average weight of the girls in the class, we can break down the calculations step-by-step:

Step 1: Calculate the total weight of the entire class
We are given that there are 60 students in the class and their average weight is 42 kg.
$\text{Total weight of the class}=\text{Total students}\times \text{Average weight}$
$\text{Total weight of the class}=60\times 42=2520\text{kg}$

Step 2: Find the number of girls in the class
There are 36 boys out of the total 60 students.
$\text{Number of girls}=60-36=24$

Step 3: Calculate the total weight of the boys
The average weight of the 36 boys is 50 kg.
$\text{Total weight of boys}=36\times 50=1800\text{kg}$

Step 4: Calculate the total weight of the girls
Subtract the total weight of the boys from the total weight of the class:
$\text{Total weight of girls}=2520-1800=720\text{kg}$

Step 5: Calculate the average weight of the girls
Divide the total weight of the girls by the number of girls:
$\text{Average weight of girls}=\frac{720}{24}=30\text{kg}$

Thus, the average weight of all the girls is 30 kg.