The lily-pad problem is a trick question.
. . And we have a trick answer for it . . .
A pond begins to develop a cover of lily pads.
The number of lily pads doubles every 3 days.
After 49 days, the pond is completely covered.
How long before the pond is one-quarter covered?
Assume the lily pads are all the same size.
"The number of lily pads doubles every 3 days."
The pond is completely covered on the 49th day.
This means that 3 days ago (46th day), the pond was half covered.
And 3 days before that, the pond was one-quarter covered.
Answer: .43 days.