4. A survey of business students who had taken the Graduate Management Admission Test (GMAT) indicated that students who have spent at least five hours studying GMAT review guides have a probability of 0.85 of scoring above 400. Students who do not review have a probability of 0.65 of scoring above 400. It has been determined that 70% of the business students review for the test.
a. Find the probability of scoring above 400.
b. Find the probability that a student who scored above 400 reviewed for the test.