Question about Using the Formula for Combinations in Probability

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Homework Statement

Each of 12 refrigerators of a certain type has been returned to a distributo because of the presence of a high pitched oscillating noise. Suppose that 5 of these 12 have defective compressors and the other 7 have less serious problems. If they are examined in random order, let X = the number among the first 6 examined that have a defective compressor. Compute the following:

P(x = 1)

E(X)

E(X^2)



Homework Equations

(n choose k) = n!/k!(n-k)!



The Attempt at a Solution

My random sample is 6, and there are 12 choose from. But I cant' seem to come up with a combination that makes any sense. (I'm just speaking specifically about trying to find P(x=1)
 

Answers and Replies

  • #2
Dick
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To get one bad compressor in a sample of 6, you have to choose 1 from the group of 5 that have bad compressors and 5 from the group of 7 that don't. Then to get the probability you divide by the number of ways to choose 6 from the 12 sample total. How do you express that in combinations language?
 

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