Tickets to the movies cost $9 for adults and $4.50 for children. If you bought 11 tickets for $72 how many adult tickets did you buy and how many children tickets did you buy?

You need to form 2 equations

1. total cost = cost of adult tickets + cost of child tickets

72 = 9a + 4.5c

2. no. of tickets = adult tickets + child tickets

11 = a + c

a = 11 - c

substitute a into eqn 1

72 = 9 (11-c) + 4.5c

72 = 99 - 9c + 4.5c

0 = 27 - 4.5c

c = 27/4.5 = 6 a = 5