Correct Answer - Option 1 : 80%

__Concept:__

The average marks = \(\rm \text{Sum of marks of all the students}\over \text{Total number of students}\)

__Calculation:__

Given the avg. marks of boys = 52 and that of girls = 42

Let the no. of boys be x and of girls be y

Sum of marks of boys = 52 x

Sum of marks of girls = 42 y

According to the question, if 50 is the combined average;

⇒ 52x + 42y = 50(x + y)

⇒ 52x + 42y = 50x + 50y

⇒ 2x = 8y

∴ x = 4y

Total number of students in the class = x + y = 4y + y = 5y

% of boys = \(\rm \frac{\text{No of boys}}{\text{Total No. of students}} \times 100\)

= \(\rm \frac{4y}{5y} \times 100\)

= 80%