Welp, this is an easy case of the Chicken and cats problem (is it called that? Idk)
Let's start with calculating what would happen if only 123 adults came. 123 x 7 = 861 dollars. That's only 32 more dollars than they made.
Now, children cost 2 dollars less than adults (that sounds weird, but I'm too lazy to correct it). Therefore, if we divide 32 by 2, we would get 16.
This means there are
16 Children
107 adults.
We double check this: 107 x 7, 16 x 5. 80 + 749 = 829. Thats the answer.
(I could be wrong, Idk)