Question 159098
Find the perimeter of the 50x50 square. 
Divide the total perimeter by 5 (for every 5 feet), this will give you the total number of posts.
A square of side s has a perimeter of 4s. 
In your case P=4(50)=200.
So you have 200 ft of border, every 5 ft a post.
{{{N=200/5=40}}}
40 posts are needed.