Answer :
On the 19th day, the lily pad covered half the pond.
The reason behind this is, If the lily pad doubles in size every day, then on the 20th day it will be twice as large as it was on the 19th day. That means it was already covering half the pond on the 19th day.
In this puzzle, the water lily is described as doubling in size every day. This means that on the first day, the lily pad is small, but by the second day, it has doubled in size. On the third day, it doubled in size again, and so on. By the 20th day, the lily pad has grown so large that it covers the entire pond.
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