ps:grid

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ps:grid

by 7806 » Sun Sep 05, 2010 11:51 am
##Alicia lives in a town whose streets are on a grid system, with all streets running east-west or north-south without breaks. Her school located on a corner, lies three blocks south and three blocks east of his home, also located on a corner. If Alicia is equally likely to choose any possible path from home to school, and if she only walks south or east, what is the probability that she will walk south for the first two blocks?

pl explain how to proceed/solve.
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by GMATGuruNY » Sun Sep 05, 2010 2:40 pm
7806 wrote:##Alicia lives in a town whose streets are on a grid system, with all streets running east-west or north-south without breaks. Her school located on a corner, lies three blocks south and three blocks east of his home, also located on a corner. If Alicia is equally likely to choose any possible path from home to school, and if she only walks south or east, what is the probability that she will walk south for the first two blocks?

pl explain how to proceed/solve.
Alicia's trip involves 3 movements south (SSS) and 3 movements east (EEE). We want to know the probability that her first 2 movements are SS. This question is no different from the following:

A bag contains three marbles labeled S and three marbles labeled E. If two marbles are randomly selected from the bag, what is the probability that both are labeled S?


3/6 * 2/5 = 1/5.
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by 7806 » Mon Sep 06, 2010 12:25 am
...just another great explanation.... simple & superb.

Thanks Mitch. :D

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by pesfunk » Sat Nov 27, 2010 8:33 pm
Cool explanation.

She has to have SSS and EEE

Total permutation = 6! / ( 3! * 3! ) = 20

Required factorial for first 2 = SS and last 4 = ( S E E E )> can be combines any ways

So S E E E = 4! / 3! = 4

the answer = 4 / 20 = 1/5
7806 wrote:...just another great explanation.... simple & superb.

Thanks Mitch. :D

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by Udaysjha » Tue Jan 31, 2012 3:22 am
GMATGuruNY wrote:
7806 wrote:##Alicia lives in a town whose streets are on a grid system, with all streets running east-west or north-south without breaks. Her school located on a corner, lies three blocks south and three blocks east of his home, also located on a corner. If Alicia is equally likely to choose any possible path from home to school, and if she only walks south or east, what is the probability that she will walk south for the first two blocks?

pl explain how to proceed/solve.
Alicia's trip involves 3 movements south (SSS) and 3 movements east (EEE). We want to know the probability that her first 2 movements are SS. This question is no different from the following:

A bag contains three marbles labeled S and three marbles labeled E. If two marbles are randomly selected from the bag, what is the probability that both are labeled S?


3/6 * 2/5 = 1/5.
Hi could u explain y u r multiplying by 2/5 instead of 2/6?

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by kwhite » Wed Feb 15, 2012 10:53 pm
why is this question so poorly worded or is it just my bad english? "what is the probability that she will walk south for the first two blocks?"

based on that, and looking at the diagram Alicia only has four options:

EE
ES
SE
SS

probability =1/4

...but i understand what the question is asking... just seems like it could be worded better.