Question

A music store is selling CD's for $5.50 and $8.50. You buy 13 CDs and spend $92.50. How many $8.50 CDs did you buy?

Answer 1

`7` CD's at $8.50
`6` CD's at $5.50

Explanation:

Define:
`color(white)("XXX")f=` number of $5.50 CD's bought
`color(white)("XXX")e=` number of $8.50 CD's bought

We are told
[1]`color(white)("XXX")f+e=13`
and
[2]`color(white)("XXX")550f+850e=9250`

Multiplying [1] by `550`
[3]`color(white)("XXX")550e+550f = 7150`

Subtracting [3] from [2]
[4]`color(white)("XXX")300e = 2100`

Dividing [4] by `300`
[5]`color(white)("XXX")e=7`

Substituting `7` for `e` in [1] (with minimal simplification)
[6]`color(white)("XXX")f=6`