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

The number of mistakes missed by both is therefore approximately

References

Pólya, G. Probabilities in Proofreading.'' Amer. Math. Monthly, 83, 42, 1976.