There are 7 marbles, including 3 green ones, so at the start, there is a 3/7 chance of getting a green marble. Assuming you did get a green one, there are now 6 marbles left, with 2 blue marbles, so there is a 2/6 chance of taking a blue marble. Given that both have to happen, you must multiply each probability, hence the total probability is 3/7 x 2/6, or a 1/7 chance. Hope this helped
