Let the number of students in class X, Y and Z be x, y and z, respectively. Total score of students in class X and Y = Total score of students in class X + Total score of students in class Y ⇒79(x+y)=83x+76y ⇒4x=3y Total score of students in class Y and Z = Total score of students in class Y + Total score of students in class Z ⇒81(y+z)=76y+85z ⇒5y=4z ∴ 20x=15y=12z Ratio of number of students in the class=