Here, the assumption is that you pull all the marbles out.
As for your question, assume that a random “draw” of 50 marbles looks like this:
[‘B’ ‘G’ ‘R’ ‘G’ ‘R’ ‘G’ ‘G’ ‘G’ ‘B’ ‘G’ ‘R’ ‘R’ ‘R’ ‘B’ ‘R’ ‘B’ ‘B’ ‘B’
‘R’ ‘B’ ‘R’ ‘B’ ‘R’ ‘R’ ‘G’ ‘B’ ‘G’ ‘R’ ‘R’ ‘R’ ‘B’ ‘B’ ‘B’ ‘R’ ‘G’ ‘B’
‘G’ ‘G’ ‘G’ ‘G’ ‘B’ ‘B’ ‘G’ ‘B’ ‘R’ ‘R’ ‘B’ ‘R’ ‘G’ ‘G’]
Now for each of these arrangement where the red marble is drawn third, you can imagine swapping the third and tenth component (the bolded components).
[‘B’ ‘G’ ‘R’ ‘G’ ‘R’ ‘G’ ‘G’ ‘G’ ‘B’ ‘G’ ‘R’ ‘R’ ‘R’ ‘B’ ‘R’ ‘B’ ‘B’ ‘B’
‘R’ ‘B’ ‘R’ ‘B’ ‘R’ ‘R’ ‘G’ ‘B’ ‘G’ ‘R’ ‘R’ ‘R’ ‘B’ ‘B’ ‘B’ ‘R’ ‘G’ ‘B’
‘G’ ‘G’ ‘G’ ‘G’ ‘B’ ‘B’ ‘G’ ‘B’ ‘R’ ‘R’ ‘B’ ‘R’ ‘G’ ‘G’]
What you should notice is that there are as many arrangements where there are red marbles in the third position vs where it’s in the 10th position. This means that the probability in Quantity A should be the the same as Quantity B because:
Required probability = (all arrangements that you require)/ (total arrangements)
and the “arrangements that you require” happens to be the same for QA and QB for the aforementioned reasons.
More rigorously, this means there’s a self inverse bijection, and so the cardinality and hence the probability (uniformly distributed) is equal.