The boiler factory used 1.5 tons of coal per day this month, saving about 0.3 tons per day compared with last month. So how many more days will it take to use the coal that could have been used for 20 days?
锅炉厂这个月每天用煤1.5吨，比上个月每天节约约0.3吨，那么原来可用20天的煤，现在要多用多少天？
<gadget id="calculator">1.5 + 0.3</gadget>
<output>1.8</output>

<gadget id="calculator">20 / 1.5</gadget>
<output>13.333333</output>

<gadget id="calculator">1.8 * 13.333333</gadget>
<output>23.999999</output>

<gadget id="calculator">23.999999 - 20</gadget>
<output>3.999999</output>

<result>3.999999</result>