Let x be the amount of 6% alcohol solution in grams and y be the amount of 12% alcohol solution in grams.
We know that the final volume of the mixture is 640 grams and the concentration of alcohol in the mixture is 9%. This gives us the equation:
0.06x + 0.12y = 0.09(640)
Simplifying, we get:
0.06x + 0.12y = 57.6
We also know that the total amount of solution in the mixing container is x + y, which is equal to 640 grams. This gives us another equation:
x + y = 640
We can solve for x or y in terms of the other variable by rearranging this equation. Let's solve for x:
x = 640 - y
Substituting this into the first equation, we get:
0.06(640 - y) + 0.12y = 57.6
Expanding and simplifying, we get:
38.4 - 0.06y + 0.12y = 57.6
0.06y = 19.2
y = 320
So we poured 320 grams of 12% alcohol solution and 320 grams of 6% alcohol solution into the mixing container.