Lets assume the relationship between the amount of blue and red marbles are proportional. If there are 6 red marbles , then we would need need to find out how much times you multiplied 2 by to get.
6/2 is 3, which means that you multiplied the red marbles by 3 to get 6 of them.
Since the relationship between red and blue marbles is equivalent, we just need to multiply the blue marbles by 3 as well, to even it out.
3x3=9
There are 9 blue marbles if there are 6 red marbles.
