prealgerbra
posted by mike .
A farmer is building a square pen 21 feet on each side. He puts one post at each corner and one post every three feet in between. how many posts will he use. please give equation

There will be one post at each corner. One each side there will be posts at 3, 6, 9, 12, 15 and 18 — 6 posts on each side counting from a corner post.
I won't do the problem for you, but does this help?