Posted by **AMY** on Friday, November 26, 2010 at 7:57pm.

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!

- FOR MATHMATE -
**MathMate**, Friday, November 26, 2010 at 8:52pm
Thank you for the follow-up. 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.

- FOR MATHMATE -
**AMY**, Friday, November 26, 2010 at 9:23pm
Yes. DId you see the total cost?? Was I right with the answer to that one? I think I was right.

- FOR MATHMATE -
**MathMate**, Friday, November 26, 2010 at 10:46pm
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.

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