Let's pick some simple points with which to set up an example for ourselves for this. Let's let the smaller radius be 1, and the larger, twice that, be 2. The radius itself is a single unit measure; in other words, it's measured as inches, feet, cm, etc., while the volume is a cubed measure. Volume is measured in inches cubed, feet cubed, cm cubed, etc. Therefore, if we have the radii measuring 1:2, we simply cube those single unit measures to find the ratio of their volumes. 1 cubed is 1, and 2 cubed is 8. So your answer for this is 1/8.
