control and dependent and independent variables

[jagspride12]

I am having a problem understanding these three things. I have a math problem I have to solve that asks for me to decide whether one i-pod price plan is better than another. Each plan has a monthly price plus a price per song downloaded. It doesn't say how many songs you would want to download per month, though. So, I was thinking that the control variable would be how many songs... But, I really don't know how to figure out the equation I need to write.

What would be the control variable?
What would be the independent variable?
What would be the dependent variable?

 

[tkhunny]

Let's see if you have been paying attention. Please provide your working definitions of these three things.