Suppose there are 50 marbles (denoted $M$) in a bag. 20 of these marbles are red (denoted $R$). 

$$ R / M = 20 / 50 = 0.4 $$ 

I interpret this as: for every 1 unit of $M$, there are $0.4$ units of $R$.

The inverse is: 

$$ M / R = 50 / 20 = 1.25 $$ 

Which is interpreted as "for every 1 unit of $R$, there are 1.25 units of $M$".

All is good until you translate these back to 'marble-speak'. The percentage suggests that "out of 50 marbles 20 are red". By analogy, inverse percentage suggests that "out of 20 red marbles 50 are marbles." The latter statement seems non-sensical to me and I wonder if there is a better way to phrase the 'marble-speak'.