Question

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?

Answer 1

x is amount of books.
x ≤ 9 if the books are 2 inches thick.
x ≤ 18 if the books are 1 inches thick.

Equation for 1 inches.
x ≤ 18

Equation for 2 inches.
2x ≤ 18

Equation for both.
x + 2y = 18
y is amount of 2 inch books

This one equation above is standard form.