A chemical fertilizer factory originally planned to complete a task in 21 days, but because it produced 4 tons more fertilizer per day, it ended up completing the task in 9 days. How many tons of fertilizer was originally planned to be produced per day?
一个化肥厂原计划21天完成一项任务，由于每天多生产化肥4吨，结果9天就完成任务，原计划每天生产化肥多少吨？
<gadget id="calculator">21 - 9</gadget>
<output>12</output>

<gadget id="calculator">9 / 12</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">4 * (3/4)</gadget>
<output>3</output>

<result>3</result>