The speed of A is 30 kilometers per hour, and the speed of B is 20 kilometers per hour. A and B set off from A to B at the same time. They arrive at B and then return to A... So they move back and forth. It is known that the distance between A and B's second head-on encounter and the point where A catches up with B for the second time is 15 kilometers away. How many kilometers is the distance between A and B?
A的速度为每小时30千米，B的速度为每小时20千米，A和B同时从甲地出发到乙地，他们先后到乙地后又返回甲地…如此往返来回运动．已知A与B第二次迎面相遇与A第二次追上B的两点相距15千米，甲、乙两地相距多少千米？
<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">2 * 2 * (2/5)</gadget>
<output>8/5 = around 1.6</output>

<gadget id="calculator">2 - (8/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">15 / (2/5)</gadget>
<output>75/2 = around 37.5</output>

<result>75/2 = around 37.5</result>