Aused 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.

What are the options