# Difference between revisions of "1991 AIME Problems/Problem 13"

## Problem

A drawer contains a mixture of red socks and blue socks, at most 1991 in all. It so happens that, when two socks are selected randomly without replacement, there is a probability of exactly $\displaystyle \frac{1}{2}$ that both are red or both are blue. What is the largest possible number of red socks in the drawer that is consistent with this data?

## Solution

Let $r_{}^{}$, and $b_{}^{}$ denote the number of red and blue socks, respectively. Also, let $t_{}^{}=r_{}^{}+b_{}^{}$. The probability $P_{}^{}$ that when two socks are drawn randomly, without replacement, both are red or both are blue is given by

$P=\frac{r(r-1)}{(r+b)(r+b-1)}+\frac{b(b-1)}{(r+b)(r+b-1)}=\frac{r(r-1)+(t-r)(t-r-1)}{t(t-1)}=\frac{1}{2}\,.$

Solving the resulting quadratic equation $r_{}^{2}-rt+t(t-1)/4=0$, for $r_{}^{}$ in terms of $t_{}^{}$, one obtains that

$r=\frac{t\pm\sqrt{t}}{2}\, .$

Now, since $r_{}^{}$ and $t_{}^{}$ are positive integers, it must be the case that $t_{}^{}=n^{2}$, with $n\in\mathbb{N}$. Hence, $r=n(n\pm 1)/2$ would correspond to the general solution. For the present case, $t\leq 1991$ and so one easily finds that $n=44$ is the largest possible integer satisfying the problem conditions.

In summary, the solution is that the maximum number of red socks is $r_{}^{}=990$.