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probability question

Discussion in 'DAT Discussions' started by msu08, Jun 3, 2008.

  1. msu08

    7+ Year Member

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    math isn't my thing, and probability certainly isn't my strong point. I constantly have issues with problems like this:
    A fair coin is tossed 6 times. What is the probability that exactly 2 heads will show?

    the answer is apparently 15/64. I think I understand the 64 is from 2^6, but that's about all i've got.
    any help/suggestions? thank youuuuu!
     
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  3. vvvv

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    Use this formular
    [​IMG]
    n
    = number of trials
    r = number of specific events you wish to
    obtain
    p = probability that the event will occur
    q = probability that the event will not occur
    (q = 1 - p, the complement of the event
     
  4. happyasaclam88

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    wow ... that formula is great! thanks so much!
     
  5. Dencology

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    can you explain how you used this formula. where do you put the numbers. and what is C and is n raised to C? how do you do the Cr?
    thanks.
    [​IMG]
    n
    = number of trials
    r = number of specific events you wish to
    obtain
    p = probability that the event will occur
    q = probability that the event will not occur
    (q = 1 - p, the complement of the event[/quote]
     
  6. arpitpatel86

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    ya can some explain the formula nad how to use it?
     
  7. smile101

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    Probability isn't my best subject either, so I have been practicing (Oh and Kaplan online and SAT math are the best sources). Here's the formula:
    nCr means choose 2 from 6 (thats your combination), now multiply by the probability of the desired event (i.e. obtaining 2 heads, so you have (1/2)^2) and multiply this by the probability of not obatining heads (means 4 tails, so you get (1/2)^4)

    15 x (1/2)^2 x (1/2)^4 = 15/64

    Try this one: For the next five days, the weather channel forecasts 70% chance of raining. What is the probability that it will rain exactly 2 days?
     
  8. msu08

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    where is the 15 coming from exactly? sorry it's a stupid question but i'm still a little confused :(
     
  9. Danny289

    Danny289 Member
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    Wait for streetwolf. :)
     
  10. Danny289

    Danny289 Member
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    Thanks, today was plan study this formula for Genetics problems.Now I can study somethging else. :)
     
  11. bumpski20

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    the 15 is coming from the n C r. the C doesn't actually equal a number. in this case we would right 6 C 2. this is what 6 C 2 actually looks like in equation form:

    n! / [r! x (n-r)!] so for this problem: 6! / [2! x (6-2)!]

    this comes out to (6 x 5 x 4 x 3 x 2 x 1) / (2 x 1 x 4 x 3 x 2 x 1)

    to avoid calculations it may be easier to do (6 x 5 x 4!) / (2 x 1 x 4!)
    now you can cancel out the 4! on the top and bottom and your left with (6 x 5) / (2 x 1) which equals 15.
     
  12. ttc23

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    that equation is great.. but it's also not too hard nor time consuming to count out all the possibilitues..

    number corresponding to the order of attempt with a head..
    12, 13, 14, 15, 16
    23, 24, 25, 26
    34, 35, 36
    45, 46
    56

    5+4+3+2+1 = 15 ;)
     
  13. Streetwolf

    Streetwolf Ultra Senior Member
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    Awesome seats at a Yankee game > SDN ;)

    Looks like everything is covered here.

    BTW ttc23 try doing that for something like (14 choose 5) :)
     
  14. ttc23

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    haha... yea, I can't do that one without formula.. I did my finite course back in high school.. and I forgot almost everything
     

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