The distance between A and B is 420 kilometers. Two cars drive from A to B at the same time. The first car travels 42 kilometers per hour. It returns to B immediately and is 84 kilometers away from B. Meet the second car, how many kilometers per hour does the second car travel?
甲乙两地之间的距离是420千米，两辆汽车同时从甲地开往乙地，第一辆汽车每小时行42干米，它到乙地立即返回，在距乙地84千米处与第二辆车相遇，第二辆车每小时行多少千米？
<gadget id="calculator">420 - 84</gadget>
<output>336</output>

<gadget id="calculator">420 + 84</gadget>
<output>504</output>

<gadget id="calculator">504 / 42</gadget>
<output>12</output>

<gadget id="calculator">336 / 12</gadget>
<output>28</output>

<result>28</result>