<p>From the BB, first edition.</p>
<p>I have no idea how to come up with a concrete number for 15; the best I did was simplify it to n/(n^2-1) = 5/k.</p>
<p>Good, you multiplied it out. Now notice how the expression is arranged. n/n^2-1=5/k. From the top part, we can see that n=5. Then plug in 5 for n on the bottom and you get 24, C which is the answer.</p>