confusing notation on calculating F/X returns returns, asking, terms

brain_wash_your

when given Beginning Spot F/X as 100Yen/$, and Ending Spot F/X as 125Yen/$


What is the "F/X" return in Yen?

How do you know when to do 125-100/100 = 25%

vs. doing the inverse

[1/125 - 1/100] / 1/100 = - 20%

I'm asking in terms of the #(the inverse thing), not the sign, which i know is negative since yen depreciated.