The distance between town A and town B is 28 kilometers. The fast and slow cars leave towns A and B in the same direction at the same time. The slow car travels in front at an average of 64 kilometers per hour, and the express train travels behind at an average of 72 kilometers per hour. After a few hours, the express train To catch up with the slow train?
甲镇与乙镇相距28千米，快慢两车同时分别从甲乙两镇同向开出，慢车在前平均每小时行64千米，快车在后平均每小时行72千米，几小时候后快车才能追上慢车？
<gadget id="calculator">72 - 64</gadget>
<output>8</output>

<gadget id="calculator">28 / 8</gadget>
<output>7/2 = around 3.5</output>

<result>7/2 = around 3.5</result>