There are 20 pens, at least how many pens can be added, and they can be divided equally among 8 students.
有20支笔，至少再加多少支，可以正好平分给8名同学．
<gadget id="calculator">8 * 3</gadget>
<output>24</output>

<gadget id="calculator">24 - 20</gadget>
<output>4</output>

<result>4</result>