The length should be 80 feet.
This is because the width is 20, since there are two sides for the width in a pen, we have to add 20+20. This would equal 40. Then we need to subtract 40 from 200. 200-40 would be 160. Then since there are two sides for the length, we need to split that in half. 160÷2 would be 80. To double check, you can do 80+80 and that would still be 160. So the length of one side would be 80. But of course, this whole thing would only be right if the pen has four sides(but since i have seen many chicken pens, there were all rectangular). I hope this helped! :)
