
Changing measurement
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?

Consider a cube : V = a^3 and S = a^2 therefore S^3/V^2 = constant
You can make the same consideration for a sphere
Note also that S^3 / V^2 has no unity
Maybe you can use this to evaluate the surface of B

Yes but it's not volume divided by surface area of vice versa, its volume over volume in proportion to surface area over surface area