Question: An online pharmacy sells an over-the-counter drug, medication X, that is used for a variety of purposes. The pharmacy has data that say that 18% of its customers are HIV positive, 9% of its HIV-positive customers buy medication X, and 3% of the customers who are not HIV positive buy medication X. Find the probability that a customer who buys medication X is HIV positive. The pharmacy can use this data to shape its marketing/advertising plan.