Question

How many 9" squares are there in a 8' x 10' area?

Answer 1

by simply doing the arithmetic:
8' by 10' = 80 square feet
= 80x144 square inches
= 11520 square inches

one 9" inch square = 81 square inches, so the number of those squares = 11520/81 = 142.22

BUT, if you are tiling an 8 by 10 ft floor with 9" squares, you will have to follow this method.

8 feet = 96 inches, so you will need 96/9 or 10.67 tiles for the short side.
but we can't buy .67 of a tile, so we will need 11 tiles along the short side

10 ft = 120 inches
number of tiles = 120/9 = 13.33
so we need 14 tiles for the long side

number of tiles = 11 x 14 or 154 tiles.

(check my arithmetic, I make errors with that)