Find the pattern

if X is 10% (1/10)
first 1/20th is taken.
Then 1/19th is taken.

if X is 100% (1/1)
first 1/2 is taken
then 1/1 is taken

if X is 33.333% (1/3)
first 1/6 is taken
then 1/5 is taken

if X is 20% (1/5)
first 1/10th is taken
then 1/9th is taken


X is a fraction of 1.
Finding the first amount taken is easy. Just double the denominator.
Finding out the 2nd amount taken is a bit tricky. or I just can't think clearly.
How can we know the 2nd amount taken just from X?