A bag contains 8 red marbles and 3 green marbles. What is the least number of marbles that you must choose without looking, to be certain that you have chosen one green marble?
To be certain that you have chosen one green marble, you must choose at least 4 marbles.
If you choose the first 3 marbles and they are all red, there will still be green marbles remaining in the bag. However, if you choose the 4th marble, it is guaranteed to be green because there are only 3 green marbles in the bag.