A bag contains 3 reds, 4 white, and 5 blue marbles. Jason begtins removing marbles from the bag at random, one at a time. Wht is the least number of marbles he must remove to be sure tht he has at least 1 marble of each color?

Ans is 10.

I knew how to get 10, but now I forgot it.

Could you help me please?