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**ANDS!** The equation in part A will determine the average cost over "t" years, yes.

Your question about which system is more cost effective needs to use this average. Think about what you have done in "English" not in "Math": if you subtract the average functions of one from the other, wont you just get the difference in the averages? The problem is, the time for both of these functions is NOT the same: one lasts 9 years, the other lasts 12 years. So if I said to you about an item in real life - "What is the better LONG TERM investment", wouldn't you evaluate each other their lifetimes?

Let me know what you get. Also see if you can interpret what the answer is in real world language, and not math-talk.