august31707 wrote:

zeenie wrote:

japped187 wrote:

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) 1330

E) 7980

!!

I have a problem with this question. actually i figure this out in this way (12C1)*(9C2)+(11C1)*(7C2)+(10C1)*(5C2)+(9C1)*(3C2).what i think is after draw 1 from the 12 and 2from the 9, so it is time to drew crews from the 11 and 7.

No, In this case you have to perform two consecutive selections i.e. selection of a experienced astronaut from 12 experienced astronauts

AND selection of two inexperienced astronauts from 9 inexperienced astronauts.

selection of a experienced astronaut from 12 experienced astronauts :- 12 C 1

AND ---------------------------------------------------------------------> Multiplication Sign

selection of two inexperienced astronauts from 9 inexperienced astronauts. 9 C 2

In the later event you will choose 2 astronauts from the pool of 9 astronauts simultaneously. Say from the five alphabets a, b, c, d, e you want select a pair of alphabet. Then you will select those alphabets in pairs such as ab, ac, ad, ae, bc, ....... this way.

So (12 C 1) AND (9 C 2) -------> (12 C 1) AND (9 C 2) -------> 432

Hope that Helps!

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