Adult tickets for the school play are $12.00 and children's tickets are $8.00. if a theatre holds 300 seats and the sold out performance brings in $3280.00, how many children and adults attended the play?

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.