There is a batch of stock in the warehouse, and the goods will be shipped into the warehouse one after another, and the same amount of goods will be shipped every day. Now use cars with the same load to transport the goods in the warehouse. If 4 cars are used every day, it will be shipped in 9 days. If 5 cars are used, it will be shipped in 6 days. If one car is used to transport the original goods in the warehouse every day, how many days will it take to finish the shipment?
仓库里原有一批存货，以后陆续运货进仓，且每天运进货物同样多．现在用载重量相同的汽车将仓库里的货物运出，如果每天用4辆汽车，则9天恰好运完，如果用5辆汽车，则6天恰好运完．如果每天用一辆汽车运出仓库里原有的货物，则需要几天运完？
<gadget id="calculator">4 * 9</gadget>
<output>36</output>

<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<gadget id="calculator">36 - 30</gadget>
<output>6</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<gadget id="calculator">6 / 3</gadget>
<output>2</output>

<gadget id="calculator">2 * 9</gadget>
<output>18</output>

<gadget id="calculator">36 - 18</gadget>
<output>18</output>

<result>18</result>