Move a batch of books, 15 books each time, half of them are moved 12 times, and the remaining 9 times are moved, so how many books are moved each time on average?
搬一批书，每次15本，12次搬一半，余下的9次搬完，那么余下的平均每次搬多少本？
<gadget id="calculator">12 / 9</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">15 * (4/3)</gadget>
<output>20</output>

<result>20</result>