posted by MathMate .
Sorry Anonymous that I deleted your post by mistake, but here's the answer to the question:
"there are four posts on each side of a square fenced-in are. How many posts are there in all?"
The corner posts are counted for both of the adjacent sides.
So the total number of posts
= 4 sides *(4-1) posts
= 12 posts.
Draw a little diagram to make sure you understand the situation.