1/8 2/9 5/12 13/18
This is already ordered by denomenator.
Now for the exact value you need to make a common factor
The denamonators are:
8= 2*2*2
9=3*3
12=2*2*3
18=2*3*3
The least common factor is:
2*2*2*3*3=72
As this number will be divisible by all of the above.
1/8 2/9 5/12 13/18 becomes
9/72 16/72 30/72 52/72
So you can see that they are already in order by value too.