The length should be 80 feet. This is because the width is 20 since there are two sides for the width in 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 assume he wants a rectangular pen. A rectangle has 2 lengths and 2 widths. The 2 lengths are equal. The 2 widths are equal. The sum of the two lengths and two widths is the perimeter. He has 200 ft, so the perimeter is 200 ft. Subtract the widths from the perimeter: 200 ft - 20 ft - 20 ft = 160 ft With 160 ft he must make two equal lengths. 160 ft/2 = 80 ft
