For some number [tex]x[/tex], there are [tex]3x[/tex] red marbles and [tex]4x[/tex] blue marbles in the bag.
So the total number of marbles is [tex]7x[/tex].
This means that the total number of marbles can be any multiple of 7.
(Note that for other cases, you would need to reduce the ratio to lowest terms and then use this method.)
