System of equations will be important here. We have two pieces of information: how many tickets were sold, and the total price.
A= adults, S = students
A + S = 155 (# of tickets)
7A + 3S = 645 (total profit)
S = 155 - A , based on the first equation
Now substitute S into the second equation.
7A + 3(155 - A) = 645
7A + 465 - 3A = 645
4A = 180
180/4 = 45 adult tickets sold.
