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2 answers:
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The function is
since she pays $80 flat, and then $0.2 for each text message.
This implies that
For the last question, we have to see how many texts it takes to pay $20: we have
So, if she sends less than 100 texts a month, it is more convenient to pay $0.2 for each text. If the sends more than 100 texts a month, it is more convenient to pay a flat fee of $20 for unlimited messages.