From A to B, (1/7) is an uphill road, (2/7) is a downhill road, and the rest are flat roads. A car goes back and forth between A and B, and travels a total of 21 kilometers uphill. How many kilometers are there between A and B?
从甲地到乙地，(1/7)是上坡路，(2/7)是下坡路，其余是平路．一辆汽车往返于甲乙两地之间，往返一次一共走了21千米的上坡路，甲乙两地相距多少千米？
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

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

<gadget id="calculator">(1/7) + (2/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">21 / (3/7)</gadget>
<output>49</output>

<result>49</result>