Alex bought 9 tickets for $30. Adult tickets cost $4 and children's cost $2. How many children's tickets did he purchase?

Answer 1

A*4+C*2=30
A+C=9

double the second equation
2A+2C=18, now subtract from eq 1

4A-2A=12
A=6
C=3

Answer 2

Let a = adult tickets
c = children's tickets

a + c = 9
4a + 2c = 30

a = 9 - c

4(9 - c) + 2c = 30
36 - 4c + 2c = -6
-2c = -6
c = 3