Your village depends for its food on a nearby fishpond. One day the wind blows a water lilly seed into the pond, and the lilly begins to grow. It doubles its size every day, and its growth is such that it will cover the entire pond (and smother the fish population) in 45 days. You are away on vacation, and the villagers depend on your vigilance and intelligence to handle their affairs. Without you present they will take action about that lilly only on regular work days, and only when the lilly covers half the pond or more. The lilly will cover half the pond days after it first starts growing. The day it reaches that size happens to be a holiday....
Hint: You don't need algebra for this problem, just common sense.