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 diffrerent crews of this type are possible?
答案是432， 谁能告诉我怎么做的么- -作者: JoshwuS 时间: 2011-7-9 21:24
12 people with previous experience in space flight， you have to choose exactly 1 person, so you have 12 ways to do that. And from the rest 9 people, you have to choose 2 of them, so it is 9*8=72, but wait, remember in this question order doesn't matter, so AB is the same as BA,(in 9*8, we assume that order matters, you choose A first and then B is one order and if choose B first and then A it is another order) then you have to divide 72 by 2, that will get you 36.
so the total ways=36*12作者: jingyunmao 时间: 2011-7-13 20:25
