Letters (probability)

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LSB: Master | Next Rank: 500 Posts; Posts: 229; Joined: Thu Nov 08, 2007 3:47 pm; Thanked: 14 times; GMAT Score:750

by LSB » Mon Sep 01, 2008 11:33 am

Q from GMAT Prep.

Is this not a dependent prob Q? Please post your response w explanation. Appreciate it guys

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: Letters

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Source: Beat The GMAT — Problem Solving | Mon Sep 01, 2008 11:33 am

LSB: Master | Next Rank: 500 Posts; Posts: 229; Joined: Thu Nov 08, 2007 3:47 pm; Thanked: 14 times; GMAT Score:750

by LSB » Mon Sep 01, 2008 1:01 pm

the attachments in this post are quite large. I am reattaching as word documents.

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Letters.doc: letters; (52.5 KiB) Downloaded 102 times

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parallel_chase: Legendary Member; Posts: 1153; Joined: Wed Jun 20, 2007 6:21 amFollowed by: 2 members; Thanked: 146 times

by parallel_chase » Mon Sep 01, 2008 1:17 pm

The probability of A letter going to A envelop and B, C, and D in wrong

A going into right envelop is 1/4
B going into wrong envelop is 2/3 ( 1-1/3)
C going into wrong envelop is 1/2
D automatically will go into the wrong envelop 1

1/4*2/3*1/2*1 = 1/12 (multiplication because everything occurs at once)

Similarly the same procedure will follow for B, C & D and the result will also be the same

1/12+1/12+1/12+1/12 = 4/12 = 1/3.

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4meonly: Legendary Member; Posts: 891; Joined: Sat Aug 16, 2008 4:21 amFollowed by: 1 member; Thanked: 27 times; GMAT Score:660(

by 4meonly » Wed Sep 03, 2008 7:25 am

Anybody, please find the flaw in my reasosning:

1. To make 1st letter with correct adress:
1/4*2/3*1/2*1 = 1/12

2. To make 2nd letter with correct adress:
3/4*1/3*1/2*1 = 1/8

3. To make 3rd letter with correct adress:
3/4*2/3*1/2*1 = 1/4

4. To make 4th letter with correct adress:
3/4*2/3*1/2*1 = 1/4

summ will be 17/24

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canuckclint: Master | Next Rank: 500 Posts; Posts: 154; Joined: Tue Aug 26, 2008 12:59 pm; Location: Canada; Thanked: 4 times

by canuckclint » Wed Sep 03, 2008 4:15 pm

4meonly wrote:Anybody, please find the flaw in my reasosning:

1. To make 1st letter with correct adress:
1/4*2/3*1/2*1 = 1/12

2. To make 2nd letter with correct adress:
3/4*1/3*1/2*1 = 1/8

3. To make 3rd letter with correct adress:
3/4*2/3*1/2*1 = 1/4

4. To make 4th letter with correct adress:
3/4*2/3*1/2*1 = 1/4

summ will be 17/24

The flaw is in 1 and multiplications in general
1 should be = 2/24

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4meonly: Legendary Member; Posts: 891; Joined: Sat Aug 16, 2008 4:21 amFollowed by: 1 member; Thanked: 27 times; GMAT Score:660(

by 4meonly » Thu Sep 04, 2008 8:00 am

1 should be = 2/24

does not this mean that 2/24 = 1/12 ?

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