If a tractor plows a piece of land, it takes 3 hours to plow (3/7) of this piece of land. According to this calculation, how many hours will it take to finish plowing this piece of land? (Solution by proportional method)
拖拉机耕一块地，3小时耕了这块地的(3/7)，照这样计算，还要多少小时才能耕完这块地？(用比例方法解)
<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(3/7) * 3</gadget>
<output>9/7 = around 1.285714</output>

<gadget id="calculator">3 - (9/7)</gadget>
<output>12/7 = around 1.714286</output>

<gadget id="calculator">(12/7) / (3/7)</gadget>
<output>4</output>

<result>4</result>