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## Homework Statement

Suppose 9 mice are available for a study of a possible carcinogen and 4 of them will form a control group (i.e. will not receive the substance). Assuming that a random sample of 4 mice are selected, what is the probability that a particular mouse, Mike Mouse will be included in the control group?

## Homework Equations

Hypergeometric

## The Attempt at a Solution

So I'm a part time tutor and a lot of the students are coming to me about this question. The way I have done it is as a Hypergeometric Distribution with A=1 (Mike Mouse) B=8 (Other Mice) n=4 x=1.

(1C1 * 8C3) / 9C4 = .4444

The students have come back and told me that it was wrong and the answer they got from the Professor was.

(4C1 * 5C3) / 9C4 = .3175

I have talked to another tutor (which is also a professor) and he did it the same way as I did. He also said that this happened last year and went to the professor last year and they said that he was right (my answer).

So now that it's come up again, I'm second guessing myself. Any input on which way is right?