Li Qiang goes back and forth between A and B. He walks 5 kilometers per hour when he goes there, and travels 30 kilometers per hour by car when he returns. It takes 3.5 hours to go back and forth. How many kilometers is the distance between A and B?
李强往返甲、乙两地，去时步行每小时行5千米，返回时乘车每小时行30千米，往返共用3.5小时，甲、乙两地相距多少千米？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/5) + (1/30)</gadget>
<output>7/30 = around 0.233333</output>

<gadget id="calculator">3.5 / (7/30)</gadget>
<output>15</output>

<result>15</result>