Please help pretty easy math problem ( ASAP)

Let's say that the number of children's tickets sold is x and the number of adult's tickets sold is y:

(1) 720 = x + y, which we can rearrange to make x = 720 - y
(2) 3000 = 3x + 5y

Now substituting the equation from (1) into (2) we get:
3000 = 3(720 - y) + 5y
3000 = 2160 - 3y + 5y
840 = 2y
y = 420

Substituting this into equation (1) we get:
x = 720 - 420 = 300

Therefor there were 300 children's tickets sold and 420 adult's tickets sold