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A committee of 3 has to be formed randomly from a group of 6

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A committee of 3 has to be formed randomly from a group of 6 [#permalink] New post 23 Aug 2007, 18:48
A committee of 3 has to be formed randomly from a group of 6 people. Tom and Mary are in this group of 6. What is the probability that Tom will be selected into the committee but Mary will not?

1/10
1/5
3/10
2/5
1/2







This question is a lot easier than I made it out to be. What do you see in the question that tips you off to the easiest method to solve? I keep confusing something in these types of probability questions. Thanks for the help!
Manager
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 [#permalink] New post 23 Aug 2007, 19:47
Brrr - I got 1/10

1st person - Tom: 1/6
2nd person - not Mary 4/5
3rd person - not Mary - 3/4

P = 1/6 * 4/5 * 3/4 = 1/10

P.S> If Tom is 2nd or 3rd - there are no such answers for this probability in the answer choices
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 [#permalink] New post 23 Aug 2007, 21:39
total committees : 6c3 = 20

committees with only tom = 6

ans : 6/20 = 3/10
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 [#permalink] New post 23 Aug 2007, 22:00
3/10 it is.
Total committes posssible = 6C3
Committee with Tom= 4C2=6

Thus 6/20= 3/10
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 [#permalink] New post 23 Aug 2007, 22:11
empty_spaces or excelgmat,
could you please explain why Committee with Tom = 6?
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 [#permalink] New post 23 Aug 2007, 22:28
Whatever wrote:
empty_spaces or excelgmat,
could you please explain why Committee with Tom = 6?


you want tom and 2 other people
2 others from a pool of 4 can be chosen in 4c2 ways = 6
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 [#permalink] New post 23 Aug 2007, 22:35
empty_spaces wrote:
you want tom and 2 other people
2 others from a pool of 4 can be chosen in 4c2 ways = 6


Ic, thanks. I forgot to exclude Mary to get 4 people =)
  [#permalink] 23 Aug 2007, 22:35
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A committee of 3 has to be formed randomly from a group of 6

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