f6數學題一問?20點

Nancy has bought 10 different Christmas presents.In how many way can she 1.divide presents into 5 group of 2?
No. of ways
= 2(5!)
= 240
2.give all presents to 2 friends such that each of her friends receives at least
3 presents?
No. of ways
= 2[10!/(3!7!) + 10!/(4!6!)] + 10!/(5!5!)
= 912


2011-09-10 12:57:16 補充：
1.divide presents into 5 group of 2?
No. of ways
= 10!/[(5!)2^5]
= 945