The water level in a reservoir is low, but doubles every day. It takes 60 days to fill the reservoir.
How long does it take for the reservoir to become half full?
REVEAL THE ANSWER
59 days. If the water level doubles every day, the reservoir on any given day was half the size the day prior. If the reservoir is full on day 60, that means it was half full on day 59, not on day 30.