I think this is best illustrated by an example:-
3 2/3 - 2 1/5 Convert to improper fraction:-
= 11/3 - 11/5
Convert to lowest common denominator which is 15:-
11/3 * 5/5 = 55/15 and 11/5 * 3/3 = 33/5
Now we subtract:-
55/15 - 33/15 = 22/15
Simplifying 22/15 = 1 7/15 <------- the answer
