Don't have time to explain, about to run out door, but perhaps someone else can explain.
Be warned, I'm not sure if this is correct, this is just how I would set it up:
I agree with this method for solving the problem. This is equating the amount of alcohol present in each of the partial solutions with the amount of alcohol present in the mixed solution. The represents the total number of ounces present in the mix after combining the two solutions, multiplied by the final percentage of 40% to arrive at the total amount of alcohol present.