Final answer:
The greatest number of cans of blue and yellow paint Marc can mix on day 3 is 3b + 3y.
Step-by-step explanation:
In order to find the greatest number of cans of blue and yellow paint Marc can mix on day 3, we need to determine the simplified ratio of blue to yellow that was used on the other two days.
Let's say the ratio is b:y. Then, on day 1, Marc mixed b cans of blue paint and y cans of yellow paint, and on day 2, Marc mixed 2b cans of blue paint and 2y cans of yellow paint, since the ratio is the same.
To find the greatest number of cans Marc can mix on day 3, we need the same ratio, so he can mix 3b cans of blue paint and 3y cans of yellow paint.
Therefore, the greatest number of cans of blue and yellow paint Marc can mix on day 3 is 3b + 3y.