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Homework Help: Binomial probability, similar to lottery problems.

  1. Jan 5, 2010 #1
    1. The problem statement, all variables and given/known data
    An opaque bag contains 10 green counters, and 20 red ones. One counter is drawn at random and not replaced: green scores one, red scores zero. Five counters are drawn.

    Find the probability of scoring 0, 1, 2, 3, 4, 5 points.

    2. Relevant equations

    3. The attempt at a solution
    I found it pretty straighforward to work out with replacement, as it is just the simple binomial probability. But when the counters aren't replaced, surely the order counts, so i tried replacing nCr with nPr though this gave me the completely wrong answer. i know this resembles the lottery problem, but i don't understand what to do when there are only 2 distinguishable groups.
  2. jcsd
  3. Jan 5, 2010 #2


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    If you score n points, that means you drew n green and 5-n red counters. What you want to do is figure out the number of ways you can do that and divide by the total number of ways you can draw five items from thirty.
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