If proofreader finds mistakes and proofreader finds mistakes, of which were also found by , how many
mistakes were missed by both and ? Assume there are a total of mistakes, so proofreader finds a Fraction
of all mistakes, and also a Fraction of the mistakes found by . Assuming these fractions are the same,
then solving for gives
Pólya, G. ``Probabilities in Proofreading.'' Amer. Math. Monthly, 83, 42, 1976.