The ratio of blue marbles to the total number of marbles is 4:5
This means that out of every 5 marbles, 4 are blue and the remaining 1 is another color. To find the fraction of blue marbles, we divide the number of blue marbles by the total number of marbles:
4 / 5 = 0.8
So, the fraction of blue marbles is 0.8 or 4/5.