Math (probability) Help Needed

Discussion in 'Archived Threads 2001-2004' started by Peter McDonald, Jan 25, 2003.

  1. Peter McDonald

    Peter McDonald Stunt Coordinator

    Joined:
    Jul 24, 2001
    Messages:
    204
    Likes Received:
    0
    Trophy Points:
    0
    I've been trying to figure this one out, but I just can't... hopefully someone here can help out [​IMG]

    There are 25 people in a group, what is the probability that at least one person has their birthday on June 19. Assuming that the probability of being born on a day is 1/365.

    The answer is .066

    Could someone please shed some light on how to do this? I would really appreciate if someone could help me out.
     
  2. Chris Derby

    Chris Derby Second Unit

    Joined:
    Oct 31, 2000
    Messages:
    371
    Likes Received:
    0
    Trophy Points:
    0
    1/365=0.002739726027397260273972602739726
    x24=0.065753424657534246575342465753425

    there is your .066

    [​IMG]
     
  3. Denward

    Denward Supporting Actor

    Joined:
    Feb 26, 2001
    Messages:
    552
    Likes Received:
    0
    Trophy Points:
    0
    Well, the easiest way to think about this is it's 1-P(zero people with that birthday). So it's 1-(364/365)^25=.066.
     
  4. Wayne Bundrick

    Wayne Bundrick Cinematographer

    Joined:
    May 17, 1999
    Messages:
    2,358
    Likes Received:
    0
    Trophy Points:
    0
    I don't miss the three years of math I had in college.
     
  5. NickSo

    NickSo Producer

    Joined:
    Jul 2, 2000
    Messages:
    4,260
    Likes Received:
    0
    Trophy Points:
    0
    Real Name:
    Nick So
    Hey peter, how old are you and where in vancouver are you from?
     
  6. Peter McDonald

    Peter McDonald Stunt Coordinator

    Joined:
    Jul 24, 2001
    Messages:
    204
    Likes Received:
    0
    Trophy Points:
    0
    Thanks for the help guys. I still don't really get it. But at least I know how to get the answer now.

    Nick, I'm 17. I'm not actually from Vancouver though, I'm from the BC interior. I just put Vancouver to remain a bit more anonymous on the net [​IMG]

    I'm in grade 12, and I have my math final this Wednesday!! I get most of it except logs and probability. I'll be going in for help on Tuesday though...

    Thanks again,
    Peter
     
  7. Andrew S

    Andrew S Stunt Coordinator

    Joined:
    Sep 30, 2001
    Messages:
    214
    Likes Received:
    0
    Trophy Points:
    0
    I'm just a 16 year-old with ZERO probabilty experience in math (that I can remember anyway)
    But why not just 24/365?
    24/365 = 0.0657
     
  8. Peter McDonald

    Peter McDonald Stunt Coordinator

    Joined:
    Jul 24, 2001
    Messages:
    204
    Likes Received:
    0
    Trophy Points:
    0
    24/365 is the same as (1/365)*24

    But why is it 24, and not 25?

    Peter
     
  9. Denward

    Denward Supporting Actor

    Joined:
    Feb 26, 2001
    Messages:
    552
    Likes Received:
    0
    Trophy Points:
    0


    With all due respect to Chris Derby, 24/365 does round to .066 but it's not the right answer.

    Here's the logic behind the correct answer. The probability of NOT having June 19 (or any specified day) as your birthday is 364/365. If there were only 2 people in the room, then the probability that neither one was born on 6/19 is 364/365 * 364/365 = (364/365)^2. So the probability of having none of 25 people born on 6/19 is (364/365)^25. The converse of having none born on 6/19 is having at least one born on 6/19 so the answer to your original question is 1-(364/365)^25 = .0662879

    The hard way to do it is add up the probability of 1 person with a 6/19 birthday + p(2 people with 6/19 birthday) + ... p(all 25 people with 6/19 birthday).
     
  10. Peter McDonald

    Peter McDonald Stunt Coordinator

    Joined:
    Jul 24, 2001
    Messages:
    204
    Likes Received:
    0
    Trophy Points:
    0
    Thanks Denward, I think I'm getting it now [​IMG]
     
  11. Chris Derby

    Chris Derby Second Unit

    Joined:
    Oct 31, 2000
    Messages:
    371
    Likes Received:
    0
    Trophy Points:
    0
    thats what i meant... [​IMG]

    i realized that my answer wasn't correct shortly after I posted it and did a little research. its been a while since i've had to do math like that. [​IMG]
     

Share This Page