what do we know?
g = 5r
If x of the 60 new marbles are green, then
g+x = r+60-x
so,
5r+x = r+60-x
4r = 60-2x
2r = 30-x
As you can see, there is no unique solution.
(old r, old g) + (x,60-x) → (new r, new g)
If x=0, (15,75)+(60,0)→(75,75)
if x=2, (14,70)+(58,2)→(72,72)
If x=4, (13,65)+(56,4) → (69,69)
and so on, right up to
If x=15, (0,0)+(30,30)→(30,30)