Let's set up an equation.
We start with $4,000 because no matter the amount of students, x, we will always have to pay this $4,000.
However, for every student x, we must pay $20.
Thus, our equation is y = 4,000 + 20x
Now, lets set this equal to 11,000, since that is the amount of money raised.
4,000 + 20x = 11000
20x = 7000
x = 350