A phone company charges $0.39 per minute for the first 10 minutes of a long-distance phone call

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Question:

A phone company charges $0.39 per minute for the first 10 minutes of a long-distance phone call and $0.12 per minute every minute after that. Find the cost function C(x) as a function of length x of the phone call in minutes. Suppose you buy a “prepaid” phone card that is planned for a single call. Find the inverse function that determines how many minutes you can talk as a function of how much you prepaid.