A factory plans to produce 570 parts. It has been done for 10 days, and the average daily production is 21. The rest will be completed in 18 days. How many parts should be produced on average per day?
一个工厂计划生产570个零件，已经做了10天，平均每天生产21个，剩下的要在18天完成，平均每天要生产多少个？
<gadget id="calculator">10 * 21</gadget>
<output>210</output>

<gadget id="calculator">570 - 210</gadget>
<output>360</output>

<gadget id="calculator">360 / 18</gadget>
<output>20</output>

<result>20</result>