Claire went into a movie theater and bought 9 bags of popcorn and 2 pretzels, costing a total of $80. Melanie went into the same movie thearter and bought 7 bags of popcorn and 4 pretzels, costing a total of $72. Write a system of equations that could be used to determine the price of each bag of popcorn and the price of each pretzel. Define the variables that you use to write the system.