Given that the length of each curtain was 28 Amos, and that the curtains were spread across the width of the Mishkan, how much of the north and south (outer) walls remained uncovered?
Rashi: Ten Amos covered the inside of the Mishkan, two Amos, the width of the two planks (on top, one on each side), leaving 16 Amos to cover the two walls, eight on each side - and two Amos (one Amah plank, and one, socket) uncovered on each side.
Riva (5), Rashbam (9): According to the opinion that the planks were only a finger wide at the top 1 , 10 Amos covered the interior width of the Mishkan, leaving nine Amos to cover each side wall, and only one Amah (sockets) was uncovered on each side. 2
Hadar Zekenim (17): The Kerashim were straight towards the interior [of the Mishkan], and tapered off on the outside. If it were opposite, the Mishkan would be more than 10 wide, and the curtains would be reach until the sockets!
The calculation ignores the finger, and also that the length of sloped planks is a bit more than their height (10). It was a miracle that heavy curtains over a large opening did not sag in the middle, and they fit flush in the corners. (If a tablecloth is longer and wider than the table it covers, the excess sticks out. The area of 10 curtains, each 30 by four, is 1200 square Amos, about 100 more than the total area of the three sides and top of the Mishkan that they covered!) Perhaps it was also a miracle that the curtains reached until the sockets. Or, the curtains were 28 long when woven, but when they hung on the Mishkan, they stretched a little due to their own weight - PF.)