Results 1 to 2 of 2

Math Help - probability

  1. #1
    MHF Contributor
    Joined
    Sep 2008
    From
    West Malaysia
    Posts
    1,261

    probability

    i am not sure where my mistake is . Can someone pls check for me .

    Q : In a test , the math marks if candidates at a certain centre are normally distributed with a mean of 55 marks and a standard deviation of 5 marks . If a sample of 5 students is chosen at random , find the probability that at least one student obtains marks of less than 60 .

    My working :
    Let X be the random variable of the math marks of the candidates ..
    X - N ( 55 , 25 )
    P ( X < 60 ) = 0.84134

    then , let Y be the random variable of the number of students obtaining marks less than 60 .
    X - B ( 5 , 0.84134 )
    therefore P ( Y ≥ 1 ) = 0.999 ----- my answer

    But the answer given is 0.5784


    Where did i go wrong ??
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Flow Master
    mr fantastic's Avatar
    Joined
    Dec 2007
    From
    Zeitgeist
    Posts
    16,948
    Thanks
    5
    Quote Originally Posted by mathaddict View Post
    i am not sure where my mistake is . Can someone pls check for me .

    Q : In a test , the math marks if candidates at a certain centre are normally distributed with a mean of 55 marks and a standard deviation of 5 marks . If a sample of 5 students is chosen at random , find the probability that at least one student obtains marks of less than 60 .

    My working :
    Let X be the random variable of the math marks of the candidates ..
    X - N ( 55 , 25 )
    P ( X < 60 ) = 0.84134

    then , let Y be the random variable of the number of students obtaining marks less than 60 .
    X - B ( 5 , 0.84134 )
    therefore P ( Y ≥ 1 ) = 0.999 ----- my answer

    But the answer given is 0.5784


    Where did i go wrong ??
    Your working is excellent. Your answer is correct.

    The given answer of 0.5784 is wrong. It's the probability that at least one student obtains marks of greater than 60.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 10
    Last Post: January 21st 2011, 11:47 AM
  2. Replies: 3
    Last Post: May 29th 2010, 07:29 AM
  3. fundamental in probability & convergence with probability 1
    Posted in the Advanced Statistics Forum
    Replies: 3
    Last Post: February 23rd 2010, 09:58 AM
  4. Replies: 1
    Last Post: February 18th 2010, 01:54 AM
  5. Replies: 3
    Last Post: December 15th 2009, 06:30 AM

Search Tags


/mathhelpforum @mathhelpforum