Question

Jana has a rectangular sheet of cardboard that measures 10 inches by 18 inches. What is the maximum number of 3-inch by 3-inch squares that she can cut from this sheet of cardboard?

Why is the answer to this question 18 and not 20?

Answer 1

She can fit 3 squares along the 10 inch side and 6 squares along the 18 inch side. When you multiply 3 by 6 you get 18.
The answer is not 20 because you cannot fit 20 3x3 squares.