Growing Water-lily

Logic Level 1

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


The answer is 19.

Jewel Jose by Jewel Jose , 18, United Kingdom

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3 solutions

Munem Shahriar
Aug 23, 2017

The pool doubles each day. So after 10 × 2 = 20 10\times2=20 days the whole pool was covered by water-lilly. The half of 2 is 2 × 0.5 = 1 2\times0.5=1 . Therefore it will be 20 − 1 = 19 20-1=19 days.

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Annie Li
Aug 23, 2017

Because on the 20th day, it is full so that means that the day before must be half the size because 1 2 \frac{1}{2} times 2 equals a whole. Also, the 'beautiful' lily sounds quite scary because it feels like as if it wants to take over or invade the whole pond. It also looks like one of those scary monsters that looks so pretty but eats you up at the end.

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Jewel Jose
Aug 22, 2017

Since the water lily doubles each day, on day 19 the pool must be half filled so that on day 20, the whole pool can be covered.

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