Here's the math: 20 *18=360 ft. sq. The 10 foot board 5.5 in. wide. Divide the feet by 5.5. (10/5.5) to determine the number of feet in a board. That number is 0.4583. Then, to figure out how many square feet a board will ver, we take the 0.4583 and multiply it by 10 (feet in the board). That gives us 4.583 sq. ft. Next, divide 360 by 4.583.