Station A and Station B are 720 kilometers apart, and two trains leave opposite to each other. After 2 hours, the distance is still 80 kilometers. It is known that the express train travels 200 kilometers per hour, and how many kilometers per hour the slow train travels.
甲、乙两站相距720千米，两列火车由两站相对开出，2小时后还相距80千米．已知快车每小时行200千米，慢车每小时行多少千米．
<gadget id="calculator">720 - 80</gadget>
<output>640</output>

<gadget id="calculator">640 / 2</gadget>
<output>320</output>

<gadget id="calculator">320 - 200</gadget>
<output>120</output>

<result>120</result>