Answer: The answer is 400 blue marbles.
Step-by-step explanation: Given that there are 560 marbles in a bag, out of which 65% are red and rest are blue.
So, number of red marbles is
and number of blue marbles is
Now, if 28 red marbles are replaced by blue marbles, the the new number of red and blue marbles will be
Now, to get 65% of the marbles blue, we need to add some more blue marbles to the bag. Let 'x' number of blue marbles are added to the bag, then
Thus, 400 blue marbles need to be added to the bag.