A batch of goods was originally planned to transport 18 tons per day, and the shipment was completed in 84 days. The actual shipment was 21 tons per day. How many days will it actually take to complete the shipment? (Solve with ratio)
一批货物，原计划每天运走18吨，84天运完，实际每天运21吨，实际要几天运完？(用比例解)
<gadget id="calculator">18 * 84</gadget>
<output>1_512</output>

<gadget id="calculator">1_512 / 21</gadget>
<output>72</output>

<result>72</result>