Finding the different combinations
<p>How do you find all the combinations of a 40 letter string?</p>

<p>I have to find how many combinations 20 <code>D</code> and 20 <code>R</code> can make.</p>

<p>as in one combination could be...</p>

<p><code>DDDDDDDDDDDDDDDDDDDDRRRRRRRRRRRRRRRRRRRR</code></p>

<p>thats 1 combination, now how do I figure out the rest?</p>
