Let the total no. of student = x
and total no. of obeise student = y
y = 20 % of x.
⇒y = 20x/100
⇒y = x/5
⇒5y = x
⇒x - 5y = 0 ---------Eqn(i)
This is the linear equation.
given that total no of student = x = 500
Therefore no. of obeise students = y = x/5 = 500/5 = 100.