Asked by Gentics problem

Unprotected skin cells mutate at a rate of 20% when exposed to 10 rads of UV radiation. A group of scientists tested whether Sun Screen X will decrease the mutation rate. 1100 cells were tested, exposed and counted. Only 200 cells had mutated, 900 cells remained unchanged. What is the x^2 number that would be calculated from this data using the null hypothesis that "sun screen X results in no significant change"?

If someone could show me how to start this problem, that'd be great. Thanks