Zhang, Li, and Zhao are all going from A to B. At 6:00 am, Zhang and Li start from A. Zhang walks 5 kilometers per hour, Li walks 4 kilometers per hour, and Zhao walks at 8:00 am. He started from A, and Zhao and Zhang arrived at B at the same time at 6 o'clock in the evening, so how long did it take for Zhao to catch up with Li?
张、李、赵三人都从甲地到乙地，上午六时，张、李二人一起从甲地出发，张每小时走5千米，李每小时走4千米，赵上午八时才从甲地出发，傍晚六时，赵、张同时到达乙地，那么赵追上李的时间是多少．
<result>12</result>