The wharf has to unload 4,000 tons of goods. It has been unloaded for 4 hours, and 320 tons per hour are unloaded. How many tons of goods are left unloaded.
码头要卸4000吨货物，已经卸了4小时，每小时卸320吨，还剩多少吨货物没卸．
<gadget id="calculator">320 * 4</gadget>
<output>1_280</output>

<gadget id="calculator">4_000 - 1_280</gadget>
<output>2_720</output>

<result>2_720</result>