Polya

Suppose you write a program and you send the source code to two of your friends, $\mathcal{A}$ and $\mathcal{B}$. Your two friends read the code and when they finish, $A$ errors are detected by $\mathcal{A}$, $B$ errors are detected by $\mathcal{B}$, and $C$ errors are detected by both. So, in total, $A+B-C$ errors are detected and can now be eliminated. We wish to estimate the number of errors that remain unnoticed and uncorrected.