A Chemistry teacher teaches a large class of high school students. She knows from past experience that the students' marks are normal with mean 70 and variance 144. Her approach to grading is to impose standards that will yield 14% A's, 20% B's, 32% C's, 20% D's and 14% F's. (a) What mark does one have to attain to get an A in her class? (b) What is the cut off mark between the C's and D's?