Mr. sanchez has 16 feet of fencing to put around a rectangular garden he wants the garden to have the greatest possible area how long should the size of the garden be

In order to enclose the greatest possible area with the fencing
that he has, Mr. Sanchez should plant the garden inside a circle
with a diameter of  5-ft 1.12-inches.  Its area will be 20.37 square feet.

If it absolutely has to be a rectangle, then he will get the greatest
possible area when it's a square.  So each side will need to be
4-ft, and the area will be 16 square feet.

I don't know how to prove either of these without calculus.
You'll just have to trust me.