Algebra.. systems with substitution

Question

Amy's school is selling tickets to the annual dance competition. On the first day of ticket sales the school sold 2 adult tickets and 4 child tickets for a total of $64. The school took in $204 on the second day by selling 12 adult tickets and 4 child tickets. What is the price each of one adult ticket and one child ticket?

Answer 1

2a + 4c = 64

12a + 4c = 204

sibtract 1st from last equation

10a = 140

a = $14 for adult tickets

4c = 64 -28 = 36

c = 36/4 = $9 for child ticket