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A optical firm purchases glass for lenses, and it is known from past experience that the variance of the refractive index of this kind of glass is 1.26x10^-4. Since it is important that the pieces of glass have nearly the same index of refraction, the firm rejects a shipment if the sample variance of 20 pieces selected at random exceeds 2x10^4. Assuming that the sample values is a random variable from a normal population, determine the probability that a shipment will be rejected even though the variance = 1.26x10^-4.

If it is complete, any advise about how to start the problem? If I had the mean at least I think I could use normal distribution, but as it is, I'm not sure how to start it with just the variance given.