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There are 9 books stacked on a shelf. The thickness of each book is either 1 inch or 2 inches.
The height of the stack of 9 books is 14 inches.
This system of equations can be used to determine x, the number of 1-inch-thick books in the stack, and y, the number of 2-inch-thick books?
What is x + y = 9
x + 2y = 14