Lets say I have two objects, Object A and Object B

If A is of volume 100 and B is of 50. If I am asked to find the surface area of B if A is 20 can I just do it normally?

Sorry, this is hard to say.

Could I do it in this way: $\displaystyle \frac{50}{100}=\frac{\text{surface area of B}}{20} $ ?

Or do I have to mess around by changing the volume e.g. like you have to change it if you are given a volume in m^3 and you need it in cm^3?