Answer: 
Step-by-step explanation:
The formula for calculate the volume of a cone is:

Where "r" is the radius and "h" is the height.
Let's calculate the volume of the entire cone before the plane cut off the upper portion. You can identify that:

Therefore, substituting into the formula, you get:

Let's calculate the volume of the upper portion. You can identify that:

Therefore, this is:

Then, the volume of the frustum is:
