Let's denote the number of red balls as $R$, yellow balls as $Y$, and green balls as $G$.

From the given conditions:

1. There are as many red balls as yellow balls $R = Y$

2. There are twice as many yellow balls as there are green ones: $Y = 2G$

Now, let's analyze each option:

Option A: "The number of red balls is equal to the sum of yellow and green balls."

• The sum of yellow and green balls is $Y + G$.
• From $R = Y$ and $Y = 2G$, we have $R = 2G$. Therefore, the sum of yellow and green balls is $Y + G = 2G + G = 3G$.
• But $R = 2G$, so $R

eq Y + G$. This option is incorrect.

Option B: "The number of red balls is double the number of green balls."

• We know $R = 2G$ from the earlier reasoning. This option is correct.

Option C: "The number of red balls is equal to yellow balls minus green balls."

• Yellow balls minus green balls is $Y - G = 2G - G = G$.
• But $R = 2G$, so $R

eq Y - G$. This option is incorrect.

Option D: "The number of red balls cannot be ascertained."

• We have already established $R = 2G$, so the number of red balls can indeed be ascertained. This option is incorrect.

Final Answer: The correct answer is: B. is double the number of green balls.