Two trains depart from two stations that are 840 kilometers apart at the same time and meet each other in 7 hours. How many kilometers per hour does one train travel 75 kilometers per hour and the other train travels?
两列火车同时从相距840千米的两站相对开出，7小时相遇，知道其中一列火车每小时行75千米，另一列火车每小时行多少千米？
<gadget id="calculator">840 / 7</gadget>
<output>120</output>

<gadget id="calculator">120 - 75</gadget>
<output>45</output>

<result>45</result>