The distance between A and B is 140 kilometers. A car drives from A to B for the whole journey (2/7). How many kilometers is the distance from the midpoint?
甲、乙两地相距140千米，一辆汽车由甲地开往乙地，行了全程的(2/7)，这时距离中点有多少千米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">(1/2) - (2/7)</gadget>
<output>3/14 = around 0.214286</output>

<gadget id="calculator">140 * (3/14)</gadget>
<output>30</output>

<result>30</result>