The average marks of boys in a class is 52 and that of girls is 42. The average marks of boys and girls combined is 50. The percentage of boys, in the class is

Let the number of boys and girls be r and y. ∴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∴ Required percentage of boys =4y5y×100%=80%