Multiple choice general knowledge math & puzzles

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?

  1. 19 days

  2. 30 days

  3. 25 days

  4. 21 days
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Since the lily doubles every day, if the pool is fully covered on day 20, then on day 19 it must have been exactly half-covered. Doubling half gives full coverage. The common mistake is thinking it's a linear growth problem, but it's exponential. Working backwards: day 20 = full, day 19 = half, day 18 = quarter, etc.

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