There are 30 exercise books, and after taking at least as many as possible, they can be distributed to 9 children on average.
有30个练习本，最少拿出多少个后，就可以平均分给9个小朋友．
<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">30 - 27</gadget>
<output>3</output>

<result>3</result>