"A criminal hides in a room with 99 innocent people. You have a lie detector that correctly classifies 95% of people. You pick someone at random, wire them up to the machine, and ask them if they are the criminal. They say 'no', but the machine goes ?ping? and says the person is lying. What is the chance that you have caught the criminal?" The possible answers are: 17%, 5%, 83%, 50%, 95%. Source: https://www.bbc.co.uk/labuk/articles/risk/ - I think it's in section 8.