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Unknown variable in Probability question

  • 13-11-2008 8:50pm
    #1
    Registered Users, Registered Users 2 Posts: 517 ✭✭✭


    Hey,

    I have been doing some probability and we have moved onto conditional probability with mutual exclusivity, independence and such like. Im pretty ok when I have the variables to the left of the = sign. However my reasoning breaks down when i have one variable and an answer but not the 2nd variable.

    For e.g.

    A bag contains 5 black and x blue marbles. One marble is drawn at random without replacement and then another drawn. If the probability of drawing 2 blue marbles is 1/6, how many blue marbles are in the bag?
    The answer is
    4
    but I dont know a good routine to get to that - can anybody help with a way to achieve this?

    Hope this makes sense, let me know if you need more info.
    Cheers.


Comments

  • Registered Users, Registered Users 2 Posts: 16,202 ✭✭✭✭Pherekydes


    The probability of the first marble being blue is x/(x+5)

    Can you write an expression for the probability that the second one is also blue? If you can, multiply these two expressions together and set equal to 1/6 and solve for x.


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