Square garden is 12 feet long on each side. put a post at each corner and every 3 feet on each side. how many posts total?

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

Answer is 16. Workings in the attachment below.

128/3reason being 5 and 1/3 = 16/316/3 • 2/3 = 32/31 and 1/3 = 4/3 32/3 • 4/3 this will equal 128/3