A factory produces a batch of parts. The original plan is to produce 900 parts per day, and it takes 40 days to complete. The actual production is 1200 parts per day. How many days did it take to complete the task less than planned? (Solve with ratio)
某工厂生产一批零件，原计划每天生产900个，40天完成，实际每天生产1200个零件．实际比计划少用多少天就能完成任务？(用比例解)
<gadget id="calculator">900 * 40</gadget>
<output>36_000</output>

<gadget id="calculator">36_000 / 1_200</gadget>
<output>30</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<result>10</result>