You have 15 marbles and a balancing scale. One marble is lighter than the others. Your teacher tells your class that the person who is able to find the lighter one in the least amount of tries wins 2 million dollars. What is the least amount of times you could use the scale to determine which one is the light one?

Only three times. First, you put seven marbles on each side of the scale. If they are equal then the marble left out is the lighter one. If not, then take the lighter side and put three of those marbles on each side of the scale. If the scale is level that means the marble left out is the lighter one. If not take the lighter side and put one marble on each side, if it is equal then the marble left out is the lighter one. If not then the side that is lighter is the marble you're looking for.