There are 3 pen holders with 8 pens on average in each pen holder, 6 pens in the first pen holder, 9 pens in the second pen holder, how many pens are there in the third pen holder.
3个笔筒平均每个笔筒有8支笔，第一个笔筒里有6支笔，第二个笔筒里有9支笔，第三个笔筒里有多少支笔．
<gadget id="calculator">8 * 3</gadget>
<output>24</output>

<gadget id="calculator">24 - 6 - 9</gadget>
<output>9</output>

<result>9</result>