GRE Geometry - Plug and Test

Just as on Algebra or Arithmetic questions, you can plug in different answer choices into appropriate parts of the question in an effort to work "backwards." This tactic is especially useful if you want to bypass difficult calculations -- and in fact, often-times the makers of the GRE try to trick you into doing complicated math when in fact you can use the plug and test method to arrive at the answer much quicker. Consider that you are asked to find the radius of a cylinder given the height and volume of the cylinder:

For method 1, the formal calculation, you can use the formula for the volume of a cylinder, but notice that to solve for r, you need to form a complicated fraction and then take the square root of that fraction. Right away you should realize that this is too much tedious calculation, and in fact there should be an easier method. In method 2, the plug and test method, pi is approximated as 3, and each of the answer choices is plugged into the volume equation in an attempt to find out which of the answer choices gives the closest approximation to the given volume. Choice 3, 2.0081, is the best answer because in the approximation calculating, the volume is computed as 36, which is very close to the value of the given volume, 38.