Have you tried any work? Start by drawing a tree diagram with the probabilities.
Mumble.com uses tests with a 5% false positive and a 10% false negative. A mandatory second testing is being done.
a. if one employee is selected at random, what is the probability that the selected employee uses drugs and tests positive twice?
b. what is the probability that the employee tests positive twice?
c. if we know that the randomly chosen employee has tested positive twice, what is the probability that he uses drugs?