Volume of a frustum two ways

You will write the volume of a frustum by the formula and as a big cone minus a small cone. Then you will turn the volume into litres and count how many buckets fill a drum.

The idea. The volume is $\dfrac{\pi h}{3}(R^2 + Rr + r^2)$. It also equals the volume of the complete cone minus the volume of the cone cut off.

Your tasks.

1. In volume, return the volume of the frustum by the formula.
2. In volume_by_cones, return the volume as the complete cone minus the small cone.
3. In litres, return the capacity of the frustum in litres, for measurements in centimetres.
4. In buckets_to_fill, return the whole number of full buckets needed to fill a drum with a volume of drum cubic centimetres. Round up.