A train travels from city A to city B for 3 hours. When returning, the speed of the train decreases by 10 kilometers per hour, and the journey takes half an hour longer. What is the distance in kilometers between A and B?
一列火车由A城开到B城行驶了3小时，返回时车速每小时减少10公里，则多行驶了半小时，那么A、B两地相距的公里数是多少
<gadget id="calculator">3.5 * 10</gadget>
<output>35</output>

<gadget id="calculator">3.5 - 3</gadget>
<output>0.5</output>

<gadget id="calculator">35 / 0.5</gadget>
<output>70</output>

<gadget id="calculator">3 * 70</gadget>
<output>210</output>

<result>210</result>