This is not a Permutations problem . . .
Presently, the ratio of red to green is: .The ratio of red marbles to green marbles in a container is 3 to 5.
How many red marbles should you add to the container with n green marbles
so that the probability of getting a red marble is 7/8?
Give your answer in terms of n.
If there are green marbles, the ratio is: .
. . and there are: . marbles in the container.
We will add red marbles.
Then there are: . red marbles
. . and: . marbles in the container.
Then we have: .