How many posts does it take to support a fence around a squa

How many posts does it take to support a fence around a square field measuring 360 feet on a side, if posts are placed every 18 feet?

*Note: this was answered as 20,but I believe that is the number of posts to support a straight fence.   

Solution

Since the side is 360 feet

so every 18 feet  

Number of post = 360 ft/18 ft = 20