Question 118945
Your line of thinking is correct and your equation is almost correct. 
Since the cost is higher for fewer number of students, it should be first in your equation. 
{{{32/(x-3) - (32/x) =2.40}}}
From here, multiply both sides by 100x(x-3) to get rid of your fractions and decimals. 
You will then get a quadratic equation.
One of the roots will be your answer.
If you need more help, post another question.