1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quick help in easy-ish probability

  1. Jan 22, 2008 #1

    rock.freak667

    User Avatar
    Homework Helper

    1. The problem statement, all variables and given/known data

    Here is the basic info. There are 24 prizes(4 cars,8 bicycles and 12 watches) to be given to 24 prize winners. A,B,C are 3 of the prize winners.
    Find
    (i)The probability that both A and B get cars given that C gets a car.
    (ii)The probability that either A or C or both gets a car
    (iii)The probability that A gets a car and B gets either a car of a bicycle
    (iv)The probability that A gets a car given then B gets either a car or a bicycle.

    2. Relevant equations



    3. The attempt at a solution

    (i)P([itex]A_c&B_c|C_c[/itex])=[tex]\frac{\frac{3}{23}*\frac{2}{22}}{\frac{4}{24}}[/tex]

    (ii)simply I got this one by finding when they both did not get cars and then 1 minus that value.

    (iii)[itex]\frac{4}{24}*\frac{3}{23}+\frac{4}{24}*\frac{8}{23}[/itex]

    (iv)This part confused me as I got a way wrong answer.(ANS=[itex]\frac{11}{69}[/itex], which I got something way smaller than that)
     
  2. jcsd
  3. Jan 23, 2008 #2

    Shooting Star

    User Avatar
    Homework Helper

    (iv) Does the sentence read "The probability that A gets a car given that B gets either a car or a bicycle."

    EDIT: correct method follows.
     
    Last edited: Jan 23, 2008
  4. Jan 23, 2008 #3

    rock.freak667

    User Avatar
    Homework Helper

    Aren't you supposed to use
    [tex]P(A|B)=\frac{P(A\cap B)}{P(B)}[/tex]
     
  5. Jan 23, 2008 #4

    Shooting Star

    User Avatar
    Homework Helper

    All right. What is P(B) in that case? More importantly, what would be P(A_int_B)?
     
  6. Jan 23, 2008 #5

    rock.freak667

    User Avatar
    Homework Helper


    B should be the probability of getting a car or a bicycle which is 4/24+8/24.

    and so P(A_int_B)=4/24*(4/24+8/24)
     
  7. Jan 23, 2008 #6

    Shooting Star

    User Avatar
    Homework Helper

    Shouldn't it be P(A_int_B)=4/24*(4/23+8/23)? Take a sec...
     
  8. Jan 23, 2008 #7

    Shooting Star

    User Avatar
    Homework Helper

    Which gives you again your old answer...

    Do you know the correct result?
     
  9. Jan 23, 2008 #8

    rock.freak667

    User Avatar
    Homework Helper

    oh it is 23 because A gets a prize and there are 23 left...stupid me.

    the answer is 11/69
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Quick help in easy-ish probability
  1. Easy Probability (Replies: 6)

  2. Easy Probability (Replies: 7)

Loading...