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.

© 1996-9

1999-05-26