Can you help me with this statistics problem?

Question

A company requires all employees to take drug test. The company can afford only the inexpensive drug test- the one with a 5% false-positive rate and a 10% false-negative rate. (This means that 5% of those who aren't using drugs test positive incorrectly and 10% of those who are using drugs test negative). Suppose that 10% of the employees in the company are using drugs and answer the following questions.
a. If one employee is chosen at random what is the probability that the employee uses drugs and tests positive?
b. If one employee is chosen at random, what is the probability that the employee does not use drugs but tests positive anyway.
c. If one employee is chosen at random, what is the probability that the employee tests positive.
d. If we know that a randomly chosen employee has tested positive, what is the probability that he uses drugs.

Please explain how to determine the probabilities.