A jar contains 20 marbles: 4 red, 6 white and 10 blue. If you remove 1 marble at a time, randomly, what is the minimum number that you must remove to be certain that you have at least 2 marble of each color?

Question

A jar contains 20 marbles: 4 red, 6 white and 10 blue. If you remove 1 marble at a time, randomly, what is the minimum number that you must remove to be certain that you have at least 2 marble of each color? 
I guessed the answer was 18 and it was correct but I don't know how to solve it. Please help!

Ms. Sue · Answer

If you remove 16, you could possibly have 10 blue and 6 white. You must remove two more in order to be certain that you have at least two of each color.