I think you might the wrong but I'll talk you through it.
The volume of the hemisphere on it's own is (2/3)πr^3.
2/3*8^3π= 341.3333...
The volume of a cone is 1/3πr^2h
1/3*8^2*20π= 426.666π
Add them together and you get a total volume of 767.99999...π
