The general idea in this type of problem is to look for common factors, then divide both numbers by that common factor. In this case there is no common factor, so it is already in simplest form.
13 over 21 cannot be simplified any further - as the only common factor between 13 and 21 is 1.
21/80 is already in its simplest form
21/5 is the simplest form.
21/46 is the simplest form.
440/21 is in its simplest form.
10/21 is in its simplest form.
21/52 is the simplest form.
13/21 is in its simplest form.
21/22 is in simplest form.
4/21 is in its simplest form.
21/10 is in its simplest form.
21/100 is in its simplest form.
21/16 is in its simplest form.