Sum up 4 and 5; result of that is 9. Then 4/9 of the marbles in the bag are blue and 5/9 are white.
One of the easier ways in which you could solve this problem would be to write and solve an equation of ratios:
blue 4 x
------- = ------ = -------
white 5 15
5x = 60, and thus x = 12. There are 12 blue marbles.
Note that 12/15= 4/5 or 4:5.
