section of underground pipeline is designed with 15 workers working 4 hours a day, which can be completed in 18 days. Later, it is required to speed up, adding 3 people a day, and increasing the working time by 1 hour a day. So, how many days can it be completed in advance?
一段地下管道设计15个工人每天工作4小时，18天可以完成，后来要求加快速度，每天增加3人，并且每天工作时间增加1小时，那么，可以提前几天完成？
<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">18 / 18 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">15 * 4 * (1/5)</gadget>
<output>12</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<result>6</result>