Space between Mishkan and Courtyard

The width of the Courtyard of the Mishkan  is 50 amos [see “A”] (Shemos 27:12). The width of the Mishkan is 10 amos [see “B”](Rashi on Shemos 26:23). The space between the sides of the Mishkan and the sides of the courtyard is the same on each side [see “C”]. The Mishkan was exactly in the middle of the courtyard. (Rashi on Shemos 27:18)

Question: How much is the space between the side of the Mishkan and the side of the courtyard?

Answer: (50 – 10) / 2 = 20 amos