The distance between A and B is 600 kilometers. A passenger car and a truck drive from A to B at the same time. The bus arrives 4 hours earlier than the truck. When the bus arrives at B, the truck travels 400 kilometers. How long does it take for the bus to complete the journey?
甲乙两地相距600千米，一列客车和一列货车同时从甲开往乙，客车比货车早到4小时，客车到乙地时，货车行了400千米．客车行完全程要用多长时间？
<gadget id="calculator">600 - 400</gadget>
<output>200</output>

<gadget id="calculator">200 / 4</gadget>
<output>50</output>

<gadget id="calculator">600 / 50</gadget>
<output>12</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<result>8</result>