From a group of 21 astronauts that includes 12 people with previous experience in space flight, a 3-person crew is to be selected so that exactly 1 person in the crew has previous experience in space flight. How many different crews of this type are possible? (A) 432 (B) 594 (C) 864 (D) 1,330 (E) 7,980 答案是A,请问这道题怎么想的呢?谢谢~
再问一下:21个人一共分7组,第一组按c12,1*c9,表示从12人中任意挑出1人,再
从9人中挑2人,那第二组不就是只能从剩下的11人中挑1人,再从剩下的7人中挑2人,
即c11,1*c7,2了吗?还是不太明白,麻烦nn在帮忙讲讲,不用考虑一共分几个组,
每一组等怎么选吗?
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