Step-by-step explanation
Since Dylan replaces the picked marble everytime, each pick is independent from the others.
From the numbers, we can see that we have 8 red marbles and, adding them all we can get the total number of marbles:
So, the probability will be the number of possible outcomes we want, which is 8 because there are 8 red marbles, divided by the total number of possible outcomes, which is 15, the total number of marbles:
Answer
The probability is 8/15.