I answered another one of your questions so I will not explain as much. ther are 12 inches in a foot and then 5280 feet in a mile so that's 1 ft/ 12 in and 1 mi/ 5280 ft. So the math is 20 in * 1 ft/ 12 in * 1 mi/ 5280 ft = .000315 mi.
In reference to the last one, do keep in mind "square" inches and whatnot mean you square things. so 1 ft/ 12 in becomes 1 ft^2/ 144 in^2. Just as a small heads up to what could be tricky.
