help with a sampling variance problem

I cannot figure out this problem for the life of me. It says:

A process produces batches of a chemical whose impurity concentration follows a normal standard distribution with a variance of 1.75. A random sample of 20 of these batches is chosen. Find the probability that the sample variance exceeds 3.10

I know this is simple but iv tried the formula (n-1)S^2/sigma^2 every way, and have even looked through the chi square table and tried to reverse engineer an answer with no luck. Can someone please help me