68 students in class 4 went on an outing organized by 2 teachers. Big taxis are limited to 30 people, and small taxis are limited to 21 people. If you rent 3 cars, how many kinds of renting methods are there?
四1班68位学生由2位老师组织去郊游．大出租车限乘30人，小出租车限乘21人．如果租3辆车，有几种租法？
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