The distance between A and B is 240 kilometers. A slow train departs from A and travels 60 kilometers per hour. At the same time, an express train departs from B and travels 90 kilometers per hour. The two vehicles travel in the same direction. The express train is behind the slow train. How many hours can the fast train catch up with the slow train?
甲、乙两地相距240千米，一列慢车从甲地出发，每小时行60千米，同时一列快车从乙地出发，每小时行90千米，两车同向行驶，快车在慢车后面，经过多少小时快车可以追上慢车？
<gadget id="calculator">90 - 60</gadget>
<output>30</output>

<gadget id="calculator">240 / 30</gadget>
<output>8</output>

<result>8</result>