How to calculate a reverse scale factor?

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%?

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Ratio and proportion. If object a is twice as large as object b, then object b is half the size of object a. If object a is $1.2$ times the size of object b, then ______. (fill in the blank) – J. M. Oct 10 '11 at 9:44
object a is 5/6 the size of object a? – Rob Oct 10 '11 at 10:10
"Object b is...", you mean. That's basically it; reciprocate the scale factor you once used to undo the scaling. – J. M. Oct 10 '11 at 10:12
sorry yes object b, ok thanks, i think i get it :) – Rob Oct 10 '11 at 10:14

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.