For this problem you need to create a system of equations. So say the variable for adult tickets is x and the variable for student tickets is y. The first equation would be for the number of tickets and that equation would look like x+y=55. The second equation would be for money and would look like 20x+16y=1020. So put 20x+16y=1020 on top of x+y=55 and subtract x+y=55.