Cell phone company y charges a $10 start up fee plus $0.10 per minute, x. cell phone company z charges $0.20 per minute, x, with no start up fee. which function represents the difference in cost between company y and company z?
a.) f(x) = -0.10x-70
b.) f(x) = -0.10x+10
c.) f(x) = 10x-0.10
d.) f(x) = 10x+0.10

B.
There is a 10 cent difference in the cost per minute, which is where the -.10x comes from. There is also a 10 dollar difference in the start up fees, which gives you the +10. 

So, if you were to choose Cell phone company Z, you would lose 10 cents per minute, but save gain 10 dollars at the beginning. -10x+10