It takes 10 hours for car A to go from city A to city B, and it takes 15 hours for car B to go from city B to city A. Car A and car B respectively start from city A and city B at the same time and drive toward each other. Meet at 400km. What is the distance in km between cities A and B?
甲车从A城到B城要用10小时，乙车从B城到A城要用用15小时，甲、乙两车分别从A、B两城同时出发，相向而行，在离B城400km处相遇．求A、B两城的距离是多少km？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

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

<gadget id="calculator">(1/10) - (1/15)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">6 * (1/30)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">2 / (1/5)</gadget>
<output>10</output>

<gadget id="calculator">400 * 10</gadget>
<output>4_000</output>

<result>4_000</result>