Brain teaser question: The number of frogs in a pond double every day. If the pond reaches maximum frog capacity on the 16th day, on what day is the pond one quarter full of frogs?
ANSWER:
It’s tempting to think that the answer is ‘4’. After all, don’t we just have to divide 16 by 4, to get the 4th day? But wait, let’s try a thought experiment first. Exactly one day before the pond was filled to capacity what fraction of the pond was full?
The frog population doubles every day. So one day before, the pond was half full with frogs.
That means, two days ago it was filled with one quarter filled with frogs. So on the 14th day–two days before the 16th day– the pond was one quarter full of frogs.
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