In a lake, there is a patch of lily pads. Every day, the patch doubles in size. If it takes 20 days for the patch to cover the entire lake, how long would it take for the patch to cover half of the lake?


Most people come up with 10 as the answer, but that is -- you guessed it -- incorrect. It's tempting to simply divide 20 in half, but that's actually not right. If the lily pad doubles in size every day, then on any given day, the lily patch was half the size the day before. So if the patch reaches the entire size of the lake on the 20th day, it means the lily pad was half the size of the lake the day before, so on day 19. The answer is 19!