15/28 is the simplest form and, by definition, the simplest form must be equivalent to other forms of the ratio.
6/15 = 2/5 in its simplest form
71/15 is in its simplest form.
15/7 is in its simplest form.
8/15 is in its simplest form.
15/16 is in its simplest form.
8/15 is in its simplest form.
15/29 is the simplest form.
11/15 is the simplest form.
14/15 is in its simplest form.
15/14 is in its simplest form.
7/15 is in its simplest form.
15/16 is in its simplest form.