There are 5 books on the bookshelf, how many ways are there to take 2 books at a time?
书架上有5本书，每次取2本有多少种取法．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>