A passenger car and a truck drive from A to B at the same time. The bus travels 40 kilometers per hour, and the truck travels 30 kilometers per hour. After arriving at B, the bus returns immediately at the same speed and meets the truck at 30 kilometers away from B. , Find the distance between A and B in kilometers?
客车和货车同时从A地开往B地，客车每小时行40千米，货车每小时行30千米，客车到达B地后按原速立即返回，在距B地30千米出与货车相遇，求A、B两地相距多少千米？
<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<gadget id="calculator">2 / 10</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">30 * (1/5)</gadget>
<output>6</output>

<gadget id="calculator">40 * 6</gadget>
<output>240</output>

<gadget id="calculator">240 - 30</gadget>
<output>210</output>

<result>210</result>