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. 17

  3. 19

  4. 9
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Since the lily doubles every day and completely covers the pool on day 20, it must have covered exactly half the pool on day 19. Working backwards, on day 18 it would have covered one-fourth, and so on. This is a classic exponential growth puzzle.

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