The magical shrinking pool math problem will remind you of high school. You're going to need some knowledge of fractions to solve this algebra word problem.
Related: The Spider And The Fly Math Problem
This video depicts the same problem as above, but instead of the pool shrinking by 1/3 and 1/2 each time, it shrinks to 1/3 and 1/2 of the previous measure. Apply that language to the problem and you'll get the same answer as is stated in this video.
This one seems a lot easier than it really is.
Proof that geometry can be really fun.