self Maths Puzzle

In the middel of a round pool lies a beatiful water-lily .The warer-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????

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self Other Question
(a-x)(b-x)(c-x)(d-x).........(z-x) = ? & why? his is just a formula for the height of an object shot straight up into the air. i found it in my math text book. height equals meters per second subtracted by 4.9 seconds squared. the puzzle is why does the formula use 4.9 instead of 9.8 and why are the seconds squared.
H=height
m=meters
s=seconds
H= ms - 4.9ss