20s-5r
----------
10r-5s
reduce to simplest form
factor out a 5:
5(4s-r)
-------
5(2r-s)
That leaves
(4s-r)/(2r-s)
nothing else factors out
----------
10r-5s
reduce to simplest form
5(4s-r)
-------
5(2r-s)
That leaves
(4s-r)/(2r-s)
nothing else factors out