In the middle of around pool lies a 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 days will half of the pool be covered by water-lily ?

A 3
B 7
C 10
D 15

Correct Answer: 3

After 19 days the pool will be covered half by the lily. By being doubled in size in a day, the water - lily will completely cover the pool after 20 days.
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