Final answer:
To be certain we have three of each color, we need to choose 173 balls from the box.
Step-by-step explanation:
To be certain we have three of each color, we need to choose enough balls from the box that guarantees this outcome.
Since there are 95 green balls and 90 red balls, we can start by choosing 85 green balls and 87 red balls.
At this point, we have a total of 172 balls. Now, if we choose one more ball, it will guarantee that we have at least three balls of each color.
So, we must choose 173 balls to be certain we have three of each color.