[Math] How to calculate a reverse scale factor

Question

I trying to scale object up and back down, I can only use a scale factor to alter the size.
I am currently scaling upwards by 1.2.

How do I calculate the scale factor I need to return the object to its' original 100%?

Answer 1

To settle this question: we always have the relation that if some object B is $p$ times larger than object A, then we can also say that object A is $\frac1{p}$ times the size of object B. In the OP's case, to undo the $\frac65 \times$ scale-up, a multiplication by $1/(6/5)=\frac56$ is needed.