We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
No - while that is a practical look at it, statistically you are talking about a compound statistical event - meaning that you have to consider the whole and not the part.
It's like the old saw about flipping a coin and getting 50/50 odd of heads or tails. Not so. Statistically, one or the other will appear 53% of the time, the other 47% of the time. That's what gamblers, good ones anyway, know and use to their advantage.
Perhaps the question is not specific enough, but given what it is, there are only 3 marbles, therefore there are 6 possible combinations on removal of the marbles.
From these, it is clear that there are 3 occurrences of B after R, though one of them has G in between. Since the question doesn't specifically state "immediately" after then the answer must be 3 and that is 50%.