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Probability of picking one of several balls of one of two colors

  1. Nov 18, 2007 #1
    An bag contains four yellow balls and two green balls. Find the probability of getting a yellow
    ball if you. . .
    (a) . . . randomly choose a ball from the urn.
    (b) . . . randomly choose and discard two balls and then choose a third.

    i know how to do part a..it's just 4/6 = 2/3
    but for part b, i cant do it because there will be three cases: discarding 2 yellow, discarding 2 green or discarding 1 yellow and 1 green but i should only get 1 answer
    ???
     
  2. jcsd
  3. Nov 18, 2007 #2

    mathman

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    For part (b) you will find it easier to do if you assume that you DON'T look at the 2 balls you discarded. In this way you will see the answer is still 2/3.
     
  4. Nov 19, 2007 #3
    but how did you get 2/3 ?
    i tried to work it but couldn't know how??
     
  5. Nov 19, 2007 #4

    mathman

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    If you don't know what the 2 discarded balls are, then the reasoning for (b) is identical to the reasoning for (a).
     
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