A passenger car travels from A to B at 50 kilometers per hour, and a truck travels from B to A at 40 kilometers per hour. Two cars start at the same time, and then meet at a place 30 kilometers away from the midpoint of A and B, find the distance between A and B.
一辆客车从甲地开往到乙地，每小时行驶50千米，一辆货车从乙地开往甲地，每小时行驶40千米．两车同时出发，然后在离甲、乙两地的中点30千米的地方相遇，求甲、乙两地间的距离．
<gadget id="calculator">30 + 30</gadget>
<output>60</output>

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<gadget id="calculator">60 / 10</gadget>
<output>6</output>

<gadget id="calculator">50 + 40</gadget>
<output>90</output>

<gadget id="calculator">6 * 90</gadget>
<output>540</output>

<result>540</result>