Although the question gives the deck's measurements as "8 - 20", there are no units associated wit these measures. Consequently, I cannot provide a useful answer.
20 x 20 = 400 square feet.
2.78 square feet.
20 x 19 = 380 square feet.
12 x 20 = 240 square feet.
20 square feet equates to 1.86 square meters.
20 x 20 = 400 square feet.
400 square feet.
Do you mean a real square with the side length of 20 feet? Then 20 feet times 20 feet is 400 square feet.
400 square feet
2.78 square feet.
13 x 20 = 260 square feet.
12 x 20 = 240 square feet.
20 x 19 = 380 square feet.
20 square meters are 215.278208 square feet.
20 (square meters) = 215.278208 square feet
20 square feet equates to 1.86 square meters.
20 square feet