Question

in a school ,4/5 of the students are boys ,and the number of girls is 100 .find the no of boy.

Answer 1

I guess the number of boys are 400

Answer 2

let the total number of students be "a"
no of girls are 100
so, 4/5 of "a" are boys 
we can write the no of girls as = total students - boys (a - 4/5 a)

a -  4/5 a = (5a - 4a)/5
=a/5

100 = a/5
a = 100 x 5
a = 500

total no of students - no of girls = no of boys
a - 100 = boys
500 - 100 = 400