A batch of coal delivered by the power plant is planned to be used up 30 tons per day, and it will be used up in 12 days. In fact, 6 tons of coal will be saved per day. How many days will it actually take? (in proportion)
发电厂运来一批煤计划每天用30吨，12天用完，实际每天节约6吨煤，实际用了多少天？(用比例)
<gadget id="calculator">30 * 12</gadget>
<output>360</output>

<gadget id="calculator">30 - 6</gadget>
<output>24</output>

<gadget id="calculator">360 / 24</gadget>
<output>15</output>

<result>15</result>