There are 19 marbles total, when you add up all the colors. There are 8 blue marbles, so that's 8 out of 19, AKA 8/19. If you draw a blue marble and don't replace it, that's one less blue marble, and one less marble in general, AKA 7/18. So, now there are 18 marbles since you have drawn the blue one and haven't replaced it. There are 7 green marbles, and the total of marbles is 18 now. So, the chances of drawing a green marble is 7/18!