What is the probability of selecting a red marble replacing it then selecting a blue
The probability of selecting a red marble and then a blue marble depends on the ratio of red to blue marbles in the collection, and whether you replace the marble after making the first selection (with replacement) or whether you keep the first marble out and only select from the remaining marbles (without replacement).
If there are N marbles in the collection, R of which are red and B of which are blue, then the probability of selecting a red marble with replacement is R/N, and the probability of selecting a blue marble after that is also B/N. These probabilities are independent, since replacing the first marble restores the original ratio of red to blue marbles.
The probability of selecting a red marble without replacement is R/N, and the probability of selecting a blue marble after that is (B-1)/(N-1), since there is now one fewer marble in the collection and one fewer blue marble to select from. These probabilities are not independent, since the outcome of the first selection affects the ratio of red to blue marbles in the remaining collection.
In either case, the probability of selecting a red marble and then a blue marble is the product of the probabilities of each individual event, since they are assumed to be independent:
P(red then blue) = P(red) x P(blue|red)
with replacement: P(red then blue) = (R/N) x (B/N)
without replacement: P(red then blue) = (R/N) x ((B-1)/(N-1))