Im not sure how to do the equation part but Sarah's dog is 20 pounds and her sisters is 40 pounds.
- - Solve:
To find the volume of a sphere:

3.14≈π
Now since its telling us to use π, then we do our formula by multiplying by π.
π=3.14 (Approximate amount)
So:

So you are correct.
When Radius=9, you plug in for R. :)
Oh. It's rounding. So, for example, depending what you were told to round to, the second problem would be 760-330=430