One day, a deli sold a total of 37 hamburgers and hot dogs. The revenue from these sales was $135.03. The hamburgers were $3.99 each and the hot dogs cost $3.15 each.

a) Write a mathematical expression for the number of burgers, $x$, and the number of hot dogs, $y$, sold that day.
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b) Write a mathematical expression for the amount of revenue from this day's sales. Use $x$ for the number of burgers and $y$ for the number of hot dogs.
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c) How many hot dogs did the store sell that day?
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