Xiaodong traveled 30% of the journey from point A to point B. According to the midpoint of the two places, there are still 4 kilometers left. How many kilometers are there between A and B?
小东从甲地到乙地，行了全程的30%，据两地的中点还有4千米．甲、乙两地相距多少千米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">4 / (1/5)</gadget>
<output>20</output>

<result>20</result>