Question 326554
Worst case scenario: you draw all 8 red marbles first, then you draw all 4 blue marbles (or all 4 green). So up to this point, you've drawn 8+4=12 marbles. To guarantee that you've drawn at least one of each color, you need to draw one more (since there are no more colors to choose from). 


So you need to draw 13 marbles to guarantee at least one of every color.