train travels from A to B, and when it travels 70% of the journey, it exceeds the midpoint by 500 kilometers. How many kilometers are there between A and B?
一列火车从甲地开往乙地，当行了全程的70%时，超过中点500千米．甲乙两地相距多少千米？
<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(7/10) - (1/2)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">500 / (1/5)</gadget>
<output>2_500</output>

<result>2_500</result>