A Chebyshev interval with a poisson distribution

[Snarf]

[SOLVED] A Chebyshev interval with a poisson distribution

Geophysicists determine the age of a zircon by counting the number of uranium fission tracks on a polished surface; the number of these uranium fission tracks on this surface follows a Possion distribution. A particular zircon is of such an age that the average number of tracks per square centimeter is seven. Give an interval that will include at least 60% of the sample values of fission track counts obtained from a large number of square centimeter samples.

I know this problem require chebyshevs theorem, but I don't have the standard deviation. How do I figure this out?

 

[EnumaElish]

Poisson distribution has a single parameter: http://en.wikipedia.org/wiki/Poisson_distribution

 

[Snarf]

I think I've figured it out. Lambda is the mean and the variance for a Poisson distribution. So SQR 100 is the standard deviation in this problem. From there its a matter of solving for k and finding the interval.