One week, a computer store sold a total of 37 computers and hard drives. The revenue from these sales was $15,900.00. The computers sold for $600.00 per unit and the hard drives sold for $180.00 per unit. For the following problems, use $x$ for the number of computers sold and $y$ for the number of hard drives sold that week.

a) Write a mathematical expression for the combined number of computers and hard drives sold that week.
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b) Write a mathematical expression for the amount of revenue from this week's sales.
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c) How many hard drives did the store sell that week?
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