Let x = number of adult tickets, and y = number of children tickets. One equation must deal with the number of tickets, and the other equation must deal with the revenue from the tickets.
Then x + y = 300 is the number of tickets
12x + 8y = 3280 is the revenue from the tickets.
Using the substitution method:
x + y = 300  ⇒  y = 300 - x  ⇒  Equation (3)
12x + 8y = 3280  ⇒  12x + 8(300-x) = 3280  ⇒  x = 220
y = 300 - x  ⇒  y = 300-220 ⇒ 80
Therefore 220 adult tickets and 80 children's tickets were sold.
