How do I calculate the volume of a frustum if I don’t know the top part of the cone?
I believe if you have the area of the base of the top part (the top of the frustum) you can compare the areas of the top and base of it, or circumferences, etc. to find the scale factor. You can also find the scale factor by comparing radii. Note that SF = value2/value1 (e.g. radius of the top part of the frustum divided by radius of the base).
You can then multiply the height of the frustum by the scale factor to get the height of the top part, from which you can find the volume, and complete to find the frustum.
Because a frustum is literally a cone/pyramid etc. with the top chopped off the two 'separate' parts are congruent, allowing you to find the scale factor.
Hope that helps! Best of luck .
P.S. If you want a more detailed explanation see this Corbettmaths video that explains it later on.