FOR MATHMATE
posted by AMY .
Media services charges $40 for a phone and 20/mth for its economy plan. It says to find a model that determines the total cost, C(t), of operationg a media Services phone for t months. I think the answer should be in this equation  y = mx+b. Not sure.
Algebra/Equations,functions  MathMate, Friday, November 26, 2010 at 11:34am
So far so good.
What values would you assign to m and b to complete the operation model?
Algebra/Equations,functions  AMY, Friday, November 26, 2010 at 12:47pm
y = 40x+20?
Algebra/Equations,functions  MathMate, Friday, November 26, 2010 at 1:33pm
Correct!
Algebra/Equations,functions  MathMate, Friday, November 26, 2010 at 1:35pm
Watch out, it's $20/month and the phone (one time only) is $40.
so the equation should be:....
Algebra/Equations,functions  AMY, Friday, November 26, 2010 at 5:19pm
not sure but will take a stab at it. y = 20x +40?
Algebra/Equations,functions  AMY, Friday, November 26, 2010 at 6:44pm
Nope that was wrong on my part. I believe the final answer is C(t) = 20t+40
Algebra/Equations,functions  AMY, Friday, November 26, 2010 at 7:55pm
I also need to find the total cost. This is what I have.
C = 20(0) +40. (0,40).
C (5) = 20 (5) + 40.
C (5) = 100 +40 = 140 (5, 140)
So the total cost is 130. IS THIS RIGHT? Thank you!

Thank you for the followup. Sorry that I didn't check your responses.
Yes, in a function, we normally assume that
C(x)=ax+b
is equivalent to
C(t)=at+b
because t is a "dummy" variable.
However in your case, t has been stated as the variable, so it is more correct to define function as:
C(t)=20t+40
Hope you understood fully why $20 per month is associated with the variable t. 
Yes. DId you see the total cost?? Was I right with the answer to that one? I think I was right.

No, I didn't.
But if total cost for t months,
C(t) = 20t + 40, then
For 0 month:
C(0) = 20(0)+40 = 40
for 4 months:
C(4) = 20(4)+40 = 120
for 5 months:
C(5) = 20(5)+40 = 140
I do not understand how you get 130 (for 5 months)!
The question has not stipulated for how many months the total cost should be calculated.