A farmer has a rectangular field that is 48 by 36, he wants to fence 16 small pens, how much fencing does he need?
Assuming he want to arrange the pens in a 4x4 square with shared fences between the pens, then he needs
(48)(5) + (36)(5) = 420
If the pens are separate, each needing 4 sides of fencing, and all 9x12, then he needs
(16)*2(9+12) = 672
If there are 16 pens, each measuring 3x36, the answer is different.
As you can see, it helps to specify exactly what you want.