What are the basic laws of probability? Provide examples and calculations.

2.46 A particular test for the presence of steroids will be used after a professional track meet. If steroids are present, the test will accurately indicate this 95% of the time. However, if steroids are not present, the test will indicate this 90% of the time (so it is wrong 10% of the time and predicts the presence of steroids). Based on past data, it is believed that 2% of athletes do use steroids. This test is administered to one athlete and is positive for steroids. What is the probability that this person actually used steroids?