A project, which is done by 12 workers, takes 15 days to complete. If it is to be completed 5 days ahead of schedule, how many workers are needed to do it? (Solve with ratio)
一项工程，由12个工人来做，需要15天完成．如果要提前5天完成，需要几个工人来做？(用比例解)
<gadget id="calculator">12 * 15</gadget>
<output>180</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<gadget id="calculator">180 / 10</gadget>
<output>18</output>

<result>18</result>