A farmer is building a square pen 21 ft on each side He puts one post at each corner and one post every 3 ft in between How many posts will he use?

Well, isn't that just lovely? Let's see here. If the farmer is building a square pen with 21 feet on each side, he will have 5 posts on each side (4 corners and 1 in the middle). So, he will use a total of 20 posts for all four sides of the pen. Just imagine those posts standing tall and proud, holding up the pen with care and precision.