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## Combination/Probability

This topic has 4 member replies
MBALA2009 Junior | Next Rank: 30 Posts
Joined
07 May 2008
Posted:
18 messages

#### Combination/Probability

Thu Jul 10, 2008 9:49 pm
There are 8 students. 4 of them are men and 4 of them are women. If 4 students are selected from the 8 students, what is the probability that the number of men is equal to the number of women?

A) 18/35
B) 16/35
C) 14/35
D) 13/35
E) 12/35

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BlueRain Senior | Next Rank: 100 Posts
Joined
13 Jul 2008
Posted:
91 messages
1
Test Date:
08.23.2008
GMAT Score:
770
Sun Jul 13, 2008 10:42 pm
8C4 = 8! / [4! x (8-4)!]
=8! / 4! / 4!
=8x7x6x5x4x3x2x1 / 4x3x2x1 / 4! (cancel 4x3x2x1 terms in 8!)
=8x7x6x5 / 4x3x2x1 (cancel 4x2 with 8, divide 6 by 3 leaves 2)
=7x2x5
=70

Most of the terms in permutation/combinations cancel due to sharing same tail terms (1x2x3xetc.), so you don't have to do the whole 8!.

Hopefully this helps.

evansbd Senior | Next Rank: 100 Posts
Joined
01 Jul 2008
Posted:
52 messages
Target GMAT Score:
700
Fri Jul 11, 2008 11:28 am
Can anyone suggest a way to do 8C4 by hand quickly?

4C2 is simple enough if you remember factorials well....but the others, not so much.

1! - 1
2! - 2
3! - 6
4! - 24
5! - 120
6! - 720
7! - 5040
8! - 40320
9! - 362880
10! - 3628800

BlueRain Senior | Next Rank: 100 Posts
Joined
13 Jul 2008
Posted:
91 messages
1
Test Date:
08.23.2008
GMAT Score:
770
Sun Jul 13, 2008 10:42 pm
8C4 = 8! / [4! x (8-4)!]
=8! / 4! / 4!
=8x7x6x5x4x3x2x1 / 4x3x2x1 / 4! (cancel 4x3x2x1 terms in 8!)
=8x7x6x5 / 4x3x2x1 (cancel 4x2 with 8, divide 6 by 3 leaves 2)
=7x2x5
=70

Most of the terms in permutation/combinations cancel due to sharing same tail terms (1x2x3xetc.), so you don't have to do the whole 8!.

Hopefully this helps.

evansbd Senior | Next Rank: 100 Posts
Joined
01 Jul 2008
Posted:
52 messages
Target GMAT Score:
700
Fri Jul 11, 2008 11:28 am
Can anyone suggest a way to do 8C4 by hand quickly?

4C2 is simple enough if you remember factorials well....but the others, not so much.

1! - 1
2! - 2
3! - 6
4! - 24
5! - 120
6! - 720
7! - 5040
8! - 40320
9! - 362880
10! - 3628800

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