What is an example of a Fermi problem?

One classic example of a Fermi problem is the question "How many piano tuners are there in Chicago?" Arriving at a good estimate to the answer requires educated guesses, for example, of the population of Chicago, the number of persons in each household, and so on.

Generally, an answer to a Fermi problem is considered correct if it is within a factor of 10 of the real value. Fermi problems are named for physicist Enrico Fermi, who had a noted aptitude for back-of-the-envelope calculations. Fermi problems are a class of questions that call for rough estimates of unknown numbers, typically without reference to relevant information. Answering a Fermi problem well requires using general knowledge to make a series of educated guesses that arrive at a final estimate.