• Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • Varsity Tutors
    Award-winning private GMAT tutoring
    Register now and save up to $200

    Available with Beat the GMAT members only code

    MORE DETAILS
    Varsity Tutors
  • Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep
  • e-gmat Exclusive Offer
    Get 300+ Practice Questions
    25 Video lessons and 6 Webinars for FREE

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep
  • EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT

Probabilty Question Help >

This topic has 2 member replies
ramkarpa Newbie | Next Rank: 10 Posts Default Avatar
Joined
04 Aug 2006
Posted:
2 messages

Probabilty Question Help >

Post Sun Aug 13, 2006 6:53 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Tanya prepared 4 different letters to be sent to 4 different addresses,For each letter she prepared an envelope with correct address. If the 4 letters are to be put in 4 envelopes at random, what is the probability that only 1 will be put in the correct address ?

    The answer is 1/3. Any ideas any one ?

    thanks

    Need free GMAT or MBA advice from an expert? Register for Beat The GMAT now and post your question in these forums!
    viksbhat Newbie | Next Rank: 10 Posts Default Avatar
    Joined
    09 Aug 2006
    Posted:
    1 messages
    Post Tue Aug 15, 2006 6:08 am
    4 letters .. 4 envelopes ..
    probability that first envelop gets correct letter . 1/4
    probability that 2nd envelop gets wrong letter 2/3
    probability that 3rd envelop gets wrong letter 1/2
    probability that 3rd envelop gets wrong letter 1/1

    total no of cases can happen 4 .

    the probability of condition to happen is 4*1/4*2/3*1/2*1/1 = 1/3

    Hope this helps.

    Please feel free to comment

    ajith Legendary Member
    Joined
    21 Sep 2006
    Posted:
    1275 messages
    Followed by:
    2 members
    Thanked:
    124 times
    Test Date:
    April 2010
    Target GMAT Score:
    740
    Post Thu Oct 19, 2006 12:47 am
    Basically This is a Derangement problem..

    Total no Ways in which u can put letters = 4! =24

    according to the question we have to put 1 letter in right place

    we can select this letter in 4c1 ways ie 4 ways..

    We have now to put all the remaining letters in wrong envelopes this can be done 2 ways (1st letter goes to 2nd 2 to 3rd 3rd to 1st or 1st to 3rd 2nd to 1st and 3rd to 2nd , these are the only ways

    so total no of ways in this condition is satisfied is 4*2=8 ways

    so the probability is 8/24= 1/3

    you can read more about derangements in http://en.wikipedia.org/wiki/Derangement or
    http://mathworld.wolfram.com/PartialDerangement.html

    Thanked by: ace_gre

    Best Conversation Starters

    1 LUANDATO 157 topics
    2 lheiannie07 87 topics
    3 Roland2rule 75 topics
    4 ardz24 64 topics
    5 AAPL 58 topics
    See More Top Beat The GMAT Members...

    Most Active Experts

    1 image description EconomistGMATTutor

    The Economist GMAT Tutor

    163 posts
    2 image description GMATGuruNY

    The Princeton Review Teacher

    133 posts
    3 image description Jay@ManhattanReview

    Manhattan Review

    128 posts
    4 image description Rich.C@EMPOWERgma...

    EMPOWERgmat

    121 posts
    5 image description Brent@GMATPrepNow

    GMAT Prep Now Teacher

    100 posts
    See More Top Beat The GMAT Experts