In a class the average of boys and girls is 'A'. The ratio of number of boys and number of girls is 4 : 1 and the average of the boys is A -1 . Find the average of girls.

A + 4 

Ratio of       Boys     :       Girls

                     4       :          1         = 5 ( sum of ratio of boys and girls ) 

 Let the average of girls = x

ATQ,

 5A = 4 (A - 1) + 1x

  x = 5A - 4A + 4

x = A + 4 

So,  the average of girls = A + 4