In a class the average of boys and girls is 'A'. The ratio of number of boys and number of girls is 3: 1 and the average of the boys is A + 1. Find the average of girls.

A - 3

                            Boys     :       Girls

                               3       :          1         = 4

         Let the average of girls = x

ATQ,

             4A = 3 (A+1) + 1x

          ⇒  x = 4A - 3A - 3

              x = A - 3