A certain farm needs to plow a piece of land, which can be plowed by 3 tractors in 18 days, but after 15 days of plowing by 3 tractors, 2 tractors are used for other purposes, so 3 cows and 1 tractor have to be used to continue plowing. The efficiency of a tractor is 6 times that of a cow, so how many days will it take to plow the land.
某农场需耕一片地，用3台拖拉机18天可耕完，结果3台拖拉机耕15天后，有2台拖拉机调作他用，于是只好改用3头牛和一台拖拉机继续耕．一台拖拉机的效率是一头牛的6倍，这样需要多少天才能耕完这片地．
<gadget id="calculator">15 / 18</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 - (5/6)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">(1/18) / 3</gadget>
<output>1/54 = around 0.018519</output>

<gadget id="calculator">(1/18) / 3 / 6</gadget>
<output>1/324 = around 0.003086</output>

<gadget id="calculator">(1/54) + (1/324)</gadget>
<output>7/324 = around 0.021605</output>

<gadget id="calculator">(1/6) / (7/324)</gadget>
<output>54/7 = around 7.714286</output>

<result>54/7 = around 7.714286</result>