The distance between A and B is 315 kilometers. The express train and the local train respectively depart from the two places at the same time, travel towards each other, and meet each other in 3.5 hours. It is known that the express train travels 50 kilometers per hour, how many kilometers per hour does the slow train travel?
甲、乙两地相距315千米，快车和慢车分别从两地同时出发，相向而行，3.5小时相遇．已知快车每小时行50千米，慢车每小时行多少千米？
<gadget id="calculator">315 / 3.5</gadget>
<output>90</output>

<gadget id="calculator">90 - 50</gadget>
<output>40</output>

<result>40</result>