70R573N: Suppose U is a population consisting of 200 marbles in which 25 of them are red. What is the probability of getting 3 red marbles in a sample of 7?
If you had a great many more marbles and 25/200 = 1/8 were red then this would be a much mor standard problem. Because each time a marble is chosen i could say that the chance of getting a red one is 1/8. the events would be independent of one another.
If that were the case then the probability of getting 3 reds would be 7C3 * (1/8)
3 * (7/8)
4 = 35 * (1/8)
3 * (7/8)
4 = 0.04007101 = about 4%
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[size=150]Let me think about the real question.[/size]
How many ways can 3 red and 4 others be chosen 25C3 * 175C4
How many combinations of 7 are there altogether 200C7
Probability = 25C3 * 175C4 / 200C7 ==> 0.038019 ==> about 3.8%
I think that maybe 3.8% is the right answer but I am not very good at probability so I want another mathematician to look at it.
SOMEONE ELSE - PLEASE TAKE A PROPER LOOK - I WANT TO KNOW HOW TO DO THIS TOO - Melody