Suppose that 5% of a given population has a disease that is difficult to detect. Call the state where the person has the disease D and the state where they don't have it N. A test for the disease is available that reports rD or rN for whether or not the person has the disease. However, it's imperfect: if the person has the disease (D), there is a 10% chance it will report they don't have it, rN. In addition, there's also a 10% chance that it will report that a person who doesn't have the disease (N) has it, rD.
What percent of reports of the disease, rD, are false positives? What percent of the reports that the person doesn't have the disease, rN, are false negatives?