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.
21/80 is already in its simplest form
21/5 is the simplest form.
440/21 is in its simplest form.
21/10 is in its simplest form.
10/21 is in its simplest form.
4/21 is in its simplest form.
21/16 is in its simplest form.
21/52 is the simplest form.
21/100 is in its simplest form.
21/46 is the simplest form.
21/22 is in simplest form.
13/21 is in its simplest form.