Fermi Problems — Definition & Examples

Here is another famous Fermi Problem: How many drops of water are in all of the oceans on Earth?

If you think there are $D$ drops, round $D$ to the nearest integer power of ten, $10^N$, and submit the value of $N$ (an integer) as your answer. Since the answer is not completely certain, any value of $N$ within 2 of the answer will be accepted as correct.