Help wit this question please

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You are a clerk at convenience store. a customer owes 11.69 for a bag of potatoes chips. she gives you a five dollar bill. how much change should the customer receive?

Tim's phone service charges $26.39 plus an additional $0.21 for each text message sent per month. if tim's phone bill was $31.64, which equation could be used to find how many text messages, x, tim sent last month?