[求助]prep2-66

amigos

 

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~

zhousaints

12xc92

llindar

其实很简单。12个有经验的人里选1个，在剩下的没经验的9个人中选2个：C12/1(12在下）*C9/1=432

amigos

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？