A telephone company offers a monthly cellular phone plan for $19.99. It includes 350 anytime minutes plus $0.25 per minute for additional minutes. The following function is used to compute the monthly cost for a​ subscriber, where x is the number of anytime minutes used.
C(x)= 19.99 if 0 350
Compute the monthly cost of the cellular phone for use of the following anytime minutes.
​(a) 225 ​(b) 420 ​(c)