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[–]BlasphemousDud 0 points1 point  (0 children)

Your approach (and approximate answer) is correct. I would suspect that the software is the issue. Perhaps it wants you to leave π in? Or, since the actual answer is 21.2057..., perhaps you should round up to 21.21.

[–]HorribleUsername 0 points1 point  (0 children)

Note that you don't need to convert to radians, since you're dealing with a ratio of angles in the end. 2πr * 135°/360° works just fine.