Probability Puzzle - Christmas Style In Knecht Ruprecht's cellar are six sealed bottles, all of which look the same. Three of the bottles contain good cider and the other three bottles contain highly poisonous lentil juice. Ruprecht's Magic Cider Test Machine (MCTM) has three compartments, a large button and a light bulb. If you place a bottle in each of the three compartments and then press the button, the MCTM starts to work. An hour later, the light bulb turns red or green: If it lights green, all three bottles contain good cider. If it shines red, not more than two bottles contain cider. Knecht Ruprecht wants to give Santa Claus a bottle of cider. How often does he have to use the MCTM in the worst case to identify a guaranteed bottle of cider?

I couldn't improve on ${6 \choose 3}-1=19$. You don't have to try the last combination because if you haven't found the three good ones yet it must be that.