Twice 20'10" is 2×20 feet + 2×10 inches = 40 feet + 20 inches. 20 inches = 12 inches + 8 inches = 1 foot + 8 inches, so 40 feet + 1 foot = 41 feet. Then we just add the inches to make 41 feet + 8 inches. (In other words, 40ft+20in=40ft+1ft+8in=41ft+8in=41'8".) That is the perimeter of the room. We need to take off the width of the door (which I have assumed is the given measurement of 2'6", the standard width of a door) so the border length is reduced by this amount making it 39'2". The question doesn’t say how wide the border is, or the height of the room, but simply asks for its length. I just guessed that 2'6" was the width of the door and wouldn’t require the border. I hope I have explained it more clearly for you.
I apologise for the typo in the original answer. The feet and inches symbols were reversed. Perhaps that’s what confused you.