Final answer:
The fraction of the paint mixture that is white paint is 2/3.
Step-by-step explanation:
To find the fraction of white paint in the mixture, we need to determine how much of the total paint is white. The total paint in the mixture is 20 red paint + 20 blue paint + white paint. Since 40 paints are accounted for (20 red + 20 blue), the remaining paint in the mixture must be white. So, the fraction of the mixture that is white paint is 100% - 40% = 60%, or 3/5. Therefore, the answer is C. 2/3.