The letters between Pascal and Fermat, which are often credited with having started probability theory, dealt mostly with the problem of points described in Exercise 40. Pascal and Fermat considered the problem of finding a fair division of stakes if the game must be called off when the first player has won $r$ games and the second player has won $s$ games, with $r<N$ and $s<N$. Let $P(r, s)$ be the probability that player A wins the game if he has already won $r$ points and player $\mathrm{B}$ has won $s$ points. Then