For a saturday matinee, adult tickets cost $4.50 and kids under 12 pay only $2.00. if 90 tickets are sold for a total of $330, how many of the tickets were adult tickets and how many were sold to kids under 12

Adult= a, kids under 12 = k

[cost of tickets] 4.50a + 2k = 330 ---(1)
[number of tickets] a+k = 90 ---(2)

(2) multiplied by 2
so 2a + 2k = 180 ---(3)

(1) minus (3)
so 2.50a = 150
a = 60
then a+k = 90
60+k = 90
so k = 30

Then 60 adult tickets and 30 kids under 12 were sold.