In the middle of a round pool lies a beautiful water-lily. The water-lily doubles in size every day. After exactly 20 days the complete pool will be covered by the lily. After how many days will half of the pool be covered by the water-lily?

The water-lily doubles in size each day. If it covers the entire pool on day 20, then on day 19 it must have covered exactly half the pool (because doubling half gives the full pool). This is a classic exponential growth problem - the key insight is that going backwards one day means halving the coverage. The distractors (10, 15, 18) come from mistakenly thinking halving the days gives half coverage, but exponential growth doesn't work that way.