Answer:
80 ft
Explanation:
1.) Since his yard is a square, the area is side^2, so take √400 to get the one side (20 ft)
2.) Then take that 20 ft and multiply it by four (or add it four times if you really wanna do that) because perimeter, which is what you need to find to get the length of fencing, is all four sides added
3.) 20×4= 80 so it's 80 ft