Two flies are placed in a quart jar. Each day the number of flies doubles. At the end of 30 days, the jar was full of flies. When was the jar half full?
Answer: Each day the fraction of the jar filled by flies is double the fraction filled on the previous day. When the jar is becomes full, the flies occupy double the fraction of the jar filled the previous day. Therefore the jar was half full the day before the 30th day, i.e., on the 29th day.
