A piece of land has (3/7) hectares, and 3 tractors can plow it, and it can be plowed in (3/4) hours. On average, how many hectares of land is plowed per tractor per hour?
一块地有(3/7)公顷，有3台拖拉机耕，(3/4)小时可以耕完．平均每台拖拉机每小时耕地多少公顷？
<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(3/7) / (3/4) / 3</gadget>
<output>4/21 = around 0.190476</output>

<result>4/21 = around 0.190476</result>