Question

Mean of weight of 100 person is 56 kg. The mean of weight of males being 58 kg and of female 48 kg then the number of males is ?

Answer 1

Let there be M males in the 100 persons. 
        M * 58 + (100 - M ) * 48 =  56 * 100
       so  10 M = 800

there are 80  male members.
there are 20 female members.

Answer 2

there are eighty males and twenty females