There are a lot of possible total amount of numbers for this ratio. But any multiple of 7 will do. So just say 7, or 14, or 21.
If you choose 7, there will be 3 red marbles and 4 blue marbles. If you choose 14, there will be 6 red marbles and 8 blue marbles. If you choose 21, there will be 9 red marbles and 12 blue marbles. And the list goes on.
