Question

Please Help:
A telephone company offers monthly cellular phone plans for $15. It includes 200 anytime minutes but has a charge of $0.25 per minute beyond 200 minutes. I really need help finding a piecewise function for this. I already wrote:
C(x) = {15 if 0 ≤ x ≤ 200; .25(x-200) + 15 if x > 200}
I don't understand why the second part is wrong because to do 15+.25x would charge the customer for all of the minutes and leave out the 200 free ones??

Answer 1

Looking at your answer your function is correct.

If the software is telling you something's wrong, it has to be with the formatting; maybe they want you to simplify the second part of the function.  If that is the case, use the distributive property:

0.25(x-200)+15
0.25*x-0.25*200+15
0.25x-50+15
0.25x-35