Oil workers laid an oil pipeline and laid the full length (2/7) in 3 days. According to this calculation, how many days will it take to finish the rest? (Solve with ratio)
石油工人铺设一条输油管道，3天铺设了全长的(2/7)，照这样计算，余下的还需几天才能铺完？(用比例解)
<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">(2/7) * 3</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">3 - (6/7)</gadget>
<output>15/7 = around 2.142857</output>

<gadget id="calculator">(15/7) / (2/7)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>