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Can someone help with a problem on the gre?

A student makes 10 one-second measurements of the disintegration of a sample of a long-lived radioactive isotope and obtains the following values:

3,0,2,1,2,4,0,1,2,5.

How long should the student count to establish the rate to an uncertainty of 1 percent?

The answer is 5000 s.

I have calculated the mean and variance of the sample to be 2 and 24, respectively, but I don't know how to proceed. Any help would be appreciated!

Thanks

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# Homework Help: Isotopes/uncertainty in measurement

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