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Q from GMATPrep - Prob, ball in board with rows of pegs

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takegmat Newbie | Next Rank: 10 Posts Default Avatar
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Q from GMATPrep - Prob, ball in board with rows of pegs

Post Sun Mar 18, 2007 3:44 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Can someone solve this please -- See the attached screenshot.

    The OA is 3/8
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    jayhawk2001 Community Manager
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    Post Sun Mar 18, 2007 4:07 pm
    Lets number the "holes" between the pegs as follows


    Row1: ____1
    Row2: ___1_2
    Row3: __1_2_3
    Row4: _1_2_3_4

    We are asked to find the number of paths that can lead to 2.

    The total number of paths = 8

    1111 1112
    1122 1123
    1222 1223
    1233 1234

    From the above, we can see that only 3 paths end in 2.

    So probability = 3/8.

    One thing to note is that we can do all of the above only because there
    is equal probability of going either left or right at a peg.

    I'm sure there is a standard binary-tree with linked nodes formula
    that can be used here. GMAT hopefully doesn't expect someone to
    remember this :-)

    takegmat Newbie | Next Rank: 10 Posts Default Avatar
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    Post Sun Mar 18, 2007 6:38 pm
    Wow, you made it look so simple. Thanks !!!

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