There are nine 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. which system of equations can be used to determine x the number of 1-inch books in the stack and y, th number of 2-inch-thick books?

We have a mix of 1 inch books and 2 inch books, and the total of inches is 14, so we can write:

1x + 2y = 14

We also have a total of 9 books, so we can also write:

x + y = 9

So our system of equations is:

1x + 2y = 14
x + y = 9

Where 'x' is the number of 1 inch books, and 'y' is the number of 2 inch books.