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?

1 Answer
Sep 6, 2015

Answer:

#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#