Answer:
The ratio of white:milk:dark is 9:8:4
Explantion:
Let w represent white chocolate , m represent milk chocolate and d represent dark chocolate
The ratio of white to not white is 3:4
Here not white means , it can be milk or dark chocolate
Then
w : (m+d) = 3:4
-----------------------------(1)
The ratio of not dark to dark is 17:4.
Again not dark means , it will be white or milk chocolate
Then,
(w+m) : d = 17:4 -----------------------------(2)
Representing (1) in fractions

4w = 3(m+d)
4w = 3m +3d
4w -3m =3d------------------------------(3)
Representing (2) in fractions

4(w+m) = 17d
4w +4m = 17d---------------------------(4)
Subtracting (3) from (4)
4w +4m = 17d
4w -3m = 3d
(-)
-------------------------------
7m = 14d
-------------------------------


m:d = 2:1
Since w:(m+d)=3:4 let's make m:d a multiple of 4.
m:d = 8:4
w : (m+d) = 3:4 = 9:12
w:m:d = 9:8:4