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. 10

  2. 19

  3. Depends on the dimensions of the swimming pool

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Classic exponential growth puzzle. If the lily doubles each day and completely covers the pool on day 20, it must have covered HALF the pool on day 19 (because 2 × half = full). This is not day 10 (that would be halving, not doubling). The pool dimensions don't matter - the doubling logic works regardless of size.

AI explanation

Since the lily doubles in size every day, whatever size it is one day before full coverage must be exactly half of the full pool. So if the pool is fully covered on day 20, it was half-covered on day 19.

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