If one pen can be exchanged for 3 ballpoint pens, and 4 ballpoint pens can be exchanged for 7 pencils, then how many pencils can be exchanged for 4 pens?
一支钢笔能换3支圆珠笔，4支圆珠笔能换7支铅笔，那么4支钢笔能换多少支铅笔．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<result>21</result>