We can write a system of equations:
7.50a + 4.50c = 3675
a + c = 650
Where a is adult tickets, and c is children tickets.
Solve this and we get (250, 400). So there were 250 adult tickets sold, and 400 children tickets sold.
I just want to point out that the ordered pair (250, 400) stands for what A and C represent. in an ordered pair, you put the letter that comes first in the alphabet. Half the time, it will be (x, y). But in this case its (a, c). This confused me when I first learned it so I just wanted to make sure it was clear