Let the number of boys and girls be x and y, respectively. ∴ According to the question, 5x+7y=85 . . . (i) 4x+9y=85 . . . (ii) From Eqs. (i) and (ii), 5x+7y=4x+9y x=2y Put the value of x in Eq. (i), 5×2y+7y=85 ⇒17y=85 ⇒y=
85
17
=5 ∴ Total number of students =x+y =2y+y=3y =3×5=15