Answer:
0.4%
Step-by-step explanation:
We have two independent events
- The athlete who is being tested for drugs actually is using steroids.
- The test went wrong indicating a false result.
The probability of 1) to happen is 0.04 because "4% of all registered athletes use steroids".
The probability of 2) ti happen is 0.1 because "The imperfect test gives positive results (indicating drug use) for 90% of all steroid-users"
Then the probability of 1) and 2) to happen is given by
=0.4%