20 young pioneers took a small boat to visit Chengxi Park. The big boat is limited to 3 people, and the small boat is limited to 2 people (there are no empty seats on each boat). How many different charter options are there? (Hint: You can think of renting only 1 big boat)
20名少先队员乘小船游城西公园，大船限乘3人，小船限乘2人(每只船上都没有空座位)．一共有多少种不同的租船方案？(提示：可以从只租1只大船想起)
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