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-   -   This is simple for you, hard for me... (http://www.physicsforums.com/showthread.php?t=15030)

mgibel Feb23-04 08:00 PM

This is simple for you, hard for me....
 
I really need to know how to do a question like this for a course I am taking. Can you help????

Out of 2 million hay fever sufferers in Canada, 82% are allergic to ragweed. If a random sample of 7 hay fever sufferers is selected,

a) what is the probability that 3 are allergic to ragweed?

b) What is the probability that 1 is allergic to ragweed?


What is the formula???

mathman Feb24-04 10:50 PM

The basic idea is to use the binomial distribution with p=.82 and q=.18.
That is expand (p+q)n into powers of p (and q).
Then the probability of exactly k yesses is {n,k}pkqn-k.
I am using {n,k} to represent n!/k!(n-k)!. In your problem, n=7 and k=1 or k=3.


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