The power plant shipped a batch of coal, planned to use 30 tons per day, and used it up in 12 days, but actually saved 6 tons of coal per day. How many days did it actually take longer than planned? (answer with ratio)
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，实际比计划多用了多少天？(用比例解答)
<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>

<gadget id="calculator">15 - 12</gadget>
<output>3</output>

<result>3</result>