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Homework Help: Probability problem without replacement

  1. May 5, 2010 #1
    R0 R0 R1 R1 R2 G0 G0 G2 B0 B2

    Two balls are drawn at random without replacement. What is the probability
    that the sum of the numbers on the two balls is 1?

    The answer is 20/90 but what is the procedure to solve this problem?

    Probably of First Draw:
    1 - (8/10) = 2/10
    Probability of Second Draw:
    1 - (6/8) = 2/8
     
    Last edited: May 5, 2010
  2. jcsd
  3. May 5, 2010 #2
    Procedure:
    List the possible ways to get a sum of "2".
    a) 0 + 2
    b) 1 + 1
    Each of these has its own probability. Find them and add them.
     
  4. May 6, 2010 #3
    To get a total of 1 you must either draw 0 then 1, or 1 then 0.

    P(0 then 1) = (5/10) (2/9)

    and P(1 then 0) = (2/10) (5/9)
     
  5. May 6, 2010 #4
    ...and that's not what you were asking! Unless that's what your EDIT was about :confused:
     
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