Answer: 48 feet (choice D)
========================================================
Explanation
- a = 16 = one leg of the right triangle
- b = unknown = the other leg of the right triangle
- c = 20 = longest side, aka hypotenuse, of the right triangle
Use the pythagorean theorem to find b
a^2+b^2 = c^2
b = sqrt(c^2 - a^2)
b = sqrt(20^2 - 16^2)
b = sqrt(400 - 256)
b = sqrt(144)
b = 12
The missing leg of this right triangle is 12 feet long. This isn't the final answer, but it does lead to the answer. Choice B is a trick answer, so be careful.
We have a right triangle with sides 12, 16 and 20. The perimeter is therefore 12+16+20 = 28+20 = 48 feet which is the amount of fencing needed. The answer is choice D.
Side note: Your teacher mentions the shortest side is 16 feet, but that's clearly not the case since it should be 12 feet instead. This seems like a typo.