[drdee]

How do you set up this story problem? Elijah sold 25 cds and dvds. He made 4 dollars off every cd and lost 1 dollar off every dvd. He made 25 dollars total. How many of each did he sell?

[ebaines]

Set up 2 equations in 2 unkowns. Let C = number of CDs and let D = number of DVDs. He sold 25 total discs, so:

C+D = 25

And he made $25 profit:
4C +(-1)D = 25

Rearrange that second equation:

4C -D = 25.

To solve, note that if you add the first equation to the second the D's cancel out, so you can find a value for C. Then plug that value for C back into either of the two equations to find D. Can you take it from here?