Great and tricky question
Bunuel! I fell into the trap of over-calculating also. Here's what I think is the solution:
First, the important parts of the premise:
1) It asks you to identify the percentage of non-ABC liquids in the bucket. Therefore, you don't need to be able to have an exact quantity.
2) Since we are adding two buckets and need to know how the two buckets mix, we DO need to know relative size of the two buckets. (e.g., how much bigger is the blue bucket than the green bucket)
With that, let's start with statement 1:
The trap here is to start calculating a bunch of hypothetical numbers or assigning tons of variables to get the answer. But, if you remember that you need to have some info on the relative size of the two buckets, you can quickly eliminate this statement as insufficient since it doesn't tell you anything about the size of the blue bucket. For example, the green bucket could be 1 liter, and the blue bucket is 1000 liters (or vis versa). Since we don't know how much C is in the green bucket or how much A or B is in the blue bucket, sizes could be off the charts different.
Eliminate A and D.
On to statement 2:
Remember here that you have to forget about statement 1. Again, this statement doesn't tell you anything about the relative size of the buckets and now you also don't have info about how the proportions of liquids ends up in the red bucket. Therefore this is insufficient.
Eliminate B.
Both statements combined:
Statement 1 tells you the ratio of A+B in Green to A+B+C in Red. Statement 2 tells you that there is no C in Green and no A or B in Blue. Therefore, you know that:
since A+B+C in red = 1.25 (A+B) in green
and C in green = 0
And A+B in blue = 0
then,
C in blue = .25(A+B) in green
Since A and B are equal portions in green,
then A = B
then C in blue = .25 (A+B) = .25 (A+A) = .25 (2A) = 0.5A
If C is half of A, but still constitutes the same percentage of the blue bucket as A of the green bucket, you know that the blue bucket must be half the size of the green bucket. Now you have your relative size.
You also know from the second statement that blue doesn't have any A or B and green doesn't have any C. So now you can find the solution.
Assume the green bucket is 1000 liters and blue bucket is 500 liters (any ratio of 2:1 would work here).
A = 100 liters
B = 100 liters
C = 50 liters
D = (1000 liters +500 liters) - (100 liters + 100 liters + 50 liters) = 1500 - 250 = 1250.
D as a percentage = 1250/1500 = 83.33%
Therefore both statements together are sufficient.