It takes 10 hours for a car to go from city A to city B, and 15 hours for a truck to go from city B to city A. These two cars depart from cities A and B at the same time, drive towards each other, and meet at a distance of 20 kilometers from city B. What is the distance in kilometers between cities A and B?
一辆小车从A城到B城需用10小时，一辆货车从B城到A城需用15小时．这两辆车分别从A、B两城同时出发，相向开出，在离B城20千米处相遇，则A、B两城相距多少千米．
<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">20 / 6</gadget>
<output>10/3 = around 3.333333</output>

<gadget id="calculator">(10/3) * 15</gadget>
<output>50</output>

<result>50</result>