Suppose a pre-employment drug screening yields the following results: among 71 subjects with positive test results, there were 21 true positives. Among 429 negative test results, there were 420 true negatives. Calculate the following conditional probabilities. Begin by constructing a contingency table.

a) a subject actually does not use drugs, given that he or she had a negative test result.

b) a subject had a negative test result, given that he or she does not use drugs.