a used bookstore also started selling used CDs and videos. In the first week the store sold a combination of 40 CDs and videos. They charged $4 CD and $6 per video and the total sales were $180. Determine the total number of CDs and videos sold

First we must make a system of equations:
c+v=40
4c+6v=180

We can solve this system of equations using substitution
 c+v=40
    -v  -v
_________
   c=40-v

4(40-v)+6v=180
 160-4x+6v=180
-160            -160
________________
        2v=20
v=10

c+v=40
c+10=40
  -10  -10
________
    c=30

So, there are 30 CDs and 10 videos.