Question

You and a friend bought the same number of CDs and the same number of DVDs at two different stores. Your store charges $6 per CD, $9 per DVD, and you paid $36. Your friends store charges $7 per CD, $8 per DVD, and he paid $37. How many CDs did you buy?

Answer 1

set up two equations

6x+9y=36
7x+8y=37

Answer 2

use one equation to solve for one variable in terms of the other, i solve for x using the first euqation

6x=36-9y
x=6-3/2y

Answer 3

plug this into the other equation

7(6-3/2y)+8y=37

42-21/2y+8y=37

(-21/2+16/2)*y=-5

-5/2*y=-5

y=2

Answer 4

now plug this into the original equations

6*x+9*2=36

6*x+18=36

6*x=18

x=3

Answer 5

now try these in the other equation to test your answer

7*3+8*2

21+16

37

Answer 6

so the answer is 3 cd's, 2 dvd's