The distance between A and B is 840 kilometers. Two trains leave opposite to each other and meet each other after 4 hours. The express train travels 125 kilometers per hour. How many kilometers per hour does the slow train travel? (Solution of column equation)
甲乙两地相距840千米，两列火车分别从两地相对开出，4小时后相遇，快车每小时行125千米，慢车每小时行多少千米？(列方程解)
<gadget id="calculator">125 * 4</gadget>
<output>500</output>

<gadget id="calculator">840 - 500</gadget>
<output>340</output>

<gadget id="calculator">340 / 4</gadget>
<output>85</output>

<result>85</result>