In a Fermi problem you estimate something like the number of piano tuners in Chicago by approximating the population of Chicago, the fraction of households with a piano, etc. But this idea works in reverse too. Say someone claims that there are N piano tuners in Chicago. To see if this is reasonable, work backwards: what would be required about the share of households with pianos and so on? If your answer seems unreasonable, the claim is probably false. #FermiProblem #ReverseFermiProblem