A person goes back and forth between A and B, riding a bicycle at a distance of 30 kilometers per hour, and walking back at a distance of 10 kilometers per hour. The round trip takes 4 hours, find the distance between A and B.
某人往返于甲、乙两地，往时骑自行车，每小时行30千米，返时步行，每小时行10千米．往返需要4小时，求甲、乙两地的距离．
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/30) + (1/10)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">4 / (2/15)</gadget>
<output>30</output>

<result>30</result>