<p>I’m confused on the concept boxes “CO4” and “CO5,” located on pages 574-575. They’re talking about combinations and probability. The two examples that are given are as follows:</p>
<p>1 (CO4): A clothing store offers 5 different shirts. . . and customers can pick any two different shirts.</p>
<p>2 (CO5): How many different ways can 2 people finish first and second in a 5-person race?</p>
<p>The second question is worded kind of weirdly, but the idea is that in the first situation, two items are picked whose sequence does not matter, and in the second, two items are picked whose sequence does matter.</p>
<p>The confusing part: It says the answers are 20 and 40 respectively, but every which way I work it, it always comes out as 10 and 20.</p>
<p>Think about it:
Five things, call them A, B, C, D, and E.</p>
<p>If order matters, these are the possibilities:
AB, AC, AD, AE, BC, BD, BE, CD, CE, DE</p>
<p>If order doesn’t matter:
AB, BA, AC, CA, AD, DA, AE, EA, BC, CB, BD, DB, BE, EB, CD, DC, CE, EC, DE, ED</p>
<p>Who’s right, me or Adam Robinson?</p>