<p>Page 476.
Number 18.</p>
<p>edit nvm fiound it.</p>
<p>there are 5 slots with 5 letters.</p>
<p>A B C D E; C can't be on either end.</p>
<p>First slot: only 4 possibilities because C can't be one of them.
skip to the last slot.
5th slot: has to be 3, why? because this is the other slot you can't have C so the remaining letters after slot 1 minus 1 = 3</p>
<p>2nd slot: 3 left.
3rd slot: 2 left
4th slot: 1 left</p>
<h2>4x3x2x1x3= 72</h2>
<p>Another way:
npr (5,5) = 120</p>
<p>since there are 120 combos for 5 slots, each letter slot is in 24 combination.
since you don't want a letter in 2 slots, you subtract 48.</p>
<p>120-48=72</p>
<p>my second explanation isn't that clear, hope the first one helps though</p>