There are 105,600 feet in twenty miles.
There is exactly 15,840 ft. in 3 miles.
Approx 3,794,100 square milles.
20
20 x 20 = 400 square feet.
Exactly 20 feet.
55
Badwater Basin and Mount Whitney
400
20 feet * 30 feet = 600 feet squared ==============
400 square feet.
20 cm = 0.656 feet.
Do you mean a real square with the side length of 20 feet? Then 20 feet times 20 feet is 400 square feet.