[ ANSWERED]
Try a few examples to verify that the doubling time
is indeed 7 days.
Day: 46
Total Water Lilies: 22
--
-
Day 0: 4
Day 7: 8
Did the water lilies double? YES
Day 10: 10
plants
Day 17: 22
plants
Did the water lilies double? APPROXIMATELY.

it's for math not bio oopsies. :)