There are 500 children in a school.For a P.T. drill they have to stand in such a manner that the number of rows is equal to number of colmns.How many children would be left out in this arrangement?

You could solve it this way:

R = # rows
C =# of columns
R and C integers
L = # left; integer

R=C ; # of rows = # of columns
then
R*C <= 500

Initially, I have 2 equations where one is an inequality
L = 500 - (R * C)

Yipes. I hope there is a better way.

Substituting R=C; you have 2 eqns and 2 unknowns

R will end up being the integer part of the sqrt(500) or 22

L=500 - 484 or 16