as it is a square all sides are equal so all sides are 12 feet long.
how many posts on one side?
First we find out how many lengths of 3 we have for 1 side:
12 / 3 = 4
And then multiply this by the amount of sides
4 * 4 = 16
We need 16 posts
1 - 1 - 1 - 1 - 1
1 1
1 1
1 1
1 - 1 - 1 - 1 - 1