Those who plowed a piece of land with a tractor in the morning (2/7). In the afternoon, less (2/5) hectares were plowed than in the morning, leaving 7.9 hectares uncultivated. How many hectares does this land have in total?
用拖拉机耕一块地，上午耕了这块地的(2/7)．下午比上午少耕(2/5)公顷，还剩7.9公顷地没有耕．这块地一共有多少公顷？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">7.9 - (2/5)</gadget>
<output>7.5</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">(2/7) + (2/7)</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 - (4/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">7.5 / (3/7)</gadget>
<output>17.5</output>

<result>17.5</result>