It took me 3 weeks to finish writing this copybook. I wrote a total of 420 brush characters. How many brush characters do I write on average every day?
我用了3个星期才把这本习字本写完．一共写了420个毛笔字，平均每天写多少个毛笔字？
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">420 / 21</gadget>
<output>20</output>

<result>20</result>