I cannot get this problem for the life of me. I've been working on it for at least an hour and have produced no work.

It is known that the incomes of subscribers to a particular magazine have a normal distribution with a standard deviation of $6,600. A random sample of 25 subscribers is taken.

a. What is the probability that the sample standard deviation of their incomes is more than $4,000?

b. What is the probability that the sample standard deviation of their incomes is less than $8,000?

Any help, even just a started, would be great.