A Problem of Water Lillies

           In India water Lillies grow extremely rapidly in the ponds. If the growth enlarged so much that each day it covered a surface double that which it filled the day before, so that at the end of the 20^th day it entirely covered the pond, in which it grew, how long would it take two water lillies of the same size at the outset and take two water lillies of the same size at the outset and at the same rate of growth to cover the same pond?

Answer: 19 days