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# Poisson - find r Watch

1. Hi,

Given that lambda = 2.1 and P(X=r)=0.1890, find the value of r.

I don't know how to isolate r, mainly because of the r! term involved. Can someone please show me how this is done.

Thanks.
2. (Original post by MonsterFromTheId)
Hi,

Given that lambda = 2.1 and P(X=r)=0.1890, find the value of r.

I don't know how to isolate r, mainly because of the r! term involved. Can someone please show me how this is done.

Thanks.
Just try a few values. It won't take long.
3. (Original post by MonsterFromTheId)
Hi,

Given that lambda = 2.1 and P(X=r)=0.1890, find the value of r.

I don't know how to isolate r, mainly because of the r! term involved. Can someone please show me how this is done.

Thanks.
Because the probability is not rational (e^(-2.1) is not rational, so the probability is not rational) the only way to do this is just by trying values for r. (0.1890 is just an approximation so you cannot really solve for r).

0.189 is a reasonably high probability. And if lambda is 2.1 you should be excepting a number near-ish 2
4. Thanks for the help guys, I appreciate it.
5. Have you considered using the table function on the calculator?

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