The average marks scored by the students of a class is 75. If the average marks scored by the girls in the same class is 90 and that of boys is 70, what is the percentage of boys in the class?

Correct option is D

Given:
The average marks of whole class=75
The average marks of girls=90
The average marks of boys=70
Formula Used:
Allegation rule to find the ratio
Solution:
Allegation rule:
Ratio of number of boys to girls= $$(C_1-C_m):(C_m-C_2)$$​​
Where $$C_1$$​ and $$C_2$$​ are averages and  $$C_m$$ is the mean of the averages.
Here $$C_1= 90 , C_2= 70$$​ and $$C_m =75$$​​
(90-75):(75-70) =15:5 =3:1
Ratio of boys to girls = 3:1
Sum of ratio = 3+1 =4
Percentage of boys in class = $$\frac{3}{4}$$​ × 100 = 75%