Question 1063116
The central angle for the 1/20th piece of the polygon is 18 degrees, so the triangle formed by the apothem and half the side has a tangent of 9 degrees (half of the triangle)=half of the side/50, the opposite over the adjacent
50 tan 9=half of the side 7.919. 
The whole side is double that, and the perimeter is 20 x that.
If I round at the end, I get 316.77 in.