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A bookstore has a clearance shelf with $3 books and $5 books. On Sat the bookstore sold 96 of the clearance books for a total of $400. How many of the $3 books were sold?
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x + y = 96, (1) (the total amount of books)
3x + 5y = 400. (2) (the "value" equation).
Solve by any method you know.