Results 1 to 4 of 4

Math Help - Probability of receiving a reply

  1. #1
    Senior Member
    Joined
    Jul 2009
    From
    Singapore
    Posts
    338

    Probability of receiving a reply

    Letters are sent by post and the probability that they are delivered in days are:
    days: 1 2 3
    1st class: 0.9 0.1 0
    2nd class: 0.1 0.6 0.3
    A secretary sends 4 letters and the receivers reply to them immediately on the same class mail. find the probability of receiving at least two replies within three days using second class post?
    P(reply in 3 days)= 0.1\times 0.1+2\times 0.1\times 0.6=0.13
    P(1 reply in 3 days)= 0.13\times (0.87)^{3}=0.0856
    P(no replies in 3 days)= 0.87^4=0.573
    P(at least 2 replies)= 1-(0.0856+0.573)=0.341
    Answer = 0.085
    What have i done wrong? I have spent quite some time over it and can't see where i have missed anything
    Thanks
    Last edited by arze; February 2nd 2010 at 03:37 PM.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Member
    Joined
    Dec 2009
    Posts
    180
    Quote Originally Posted by arze View Post
    Letters are sent by post and the probability that they are delivered in days are:
    days: 1 2 3
    1st class: 0.9 0.1 0
    2nd class: 0.1 0.6 0.3
    A secretary sends 4 letters and the receivers reply to them immediately on the same class mail. find the probability of receiving at least two replies within three days using second class post?
    P(reply in 3 days)= 0.1\times 0.1+2\times 0/1\times 0.6=0.13 <= did you mean 2*0.1*0.6?
    P(1 reply in 3 days)= 0.13\times (0.87)^{3}=0.0856
    P(no replies in 3 days)= 0.87^4=0.573
    P(at least 2 replies)= 1-(0.0856+0.573)=0.341
    Answer = 0.085
    What have i done wrong? I have spent quite some time over it and can't see where i have missed anything
    Thanks
    What does that mean?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Senior Member
    Joined
    Jul 2009
    From
    Singapore
    Posts
    338
    sorry typing error, i have edited it
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    Joined
    Dec 2009
    Posts
    3,120
    Thanks
    1
    Quote Originally Posted by arze View Post
    Letters are sent by post and the probability that they are delivered in days are:
    days: 1 2 3
    1st class: 0.9 0.1 0
    2nd class: 0.1 0.6 0.3
    A secretary sends 4 letters and the receivers reply to them immediately on the same class mail. find the probability of receiving at least two replies within three days using second class post?
    P(reply in 3 days)= 0.1\times 0.1+2\times 0.1\times 0.6=0.13
    P(1 reply in 3 days)= 0.13\times (0.87)^{3}=0.0856 this is the probability of a reply to only one specific letter
    P(no replies in 3 days)= 0.87^4=0.573
    P(at least 2 replies)= 1-(0.0856+0.573)=0.341
    Answer = 0.085
    What have i done wrong? I have spent quite some time over it and can't see where i have missed anything
    Thanks
    P(reply to 1st letter)= (0.13)0.87^3

    P(reply to 2nd letter)= (0.13)0.87^3

    P(reply to 3rd letter)= (0.13)0.87^3

    P(reply to 4th letter)= (0.13)0.87^3

    P(no replies)= 0.87^4

    P(at most 1 reply)= 4[(0.13)0.87^3]+0.87^4=0.573+0.342=0.915

    P(at least 2 replies)= 1-0.915=0.085
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 0
    Last Post: May 22nd 2009, 11:12 AM
  2. Please reply my exam is tomorrow!!!!!!!!!
    Posted in the Algebra Forum
    Replies: 8
    Last Post: May 17th 2009, 11:27 AM
  3. Replies: 2
    Last Post: November 15th 2007, 08:38 AM

Search Tags


/mathhelpforum @mathhelpforum