2.20. All athletes at the Olympic games are tested for performance-enhancing steroid drug use. The imperfect test gives positive results (indicating drug use) for 90% of all steroid-users but also (and incorrectly) for 2% of those who do not use steroids. Suppose that 5% of all registered athletes use steroids. If an athlete is tested negative, what is the probability that he/she uses steroids?

2.20. All athletes at the Olympic games are tested for performance-enhancing steroid drug use.
The imperfect test gives positive results (indicating drug use) for 90% of all steroid-users
but also (and incorrectly) for 2% of those who do not use steroids. Suppose that 5% of all
registered athletes use steroids. If an athlete is tested negative, what is the probability that
he/she uses steroids?