45 and 28 have no common factor. In other words, you can't simplify that.
No.
No.
12/45 = 4/15
Yes.
45/36 = 5/4
45/75 = 3/5
45/113 is already in its simplest form
45/64 is already in its simplest form
28/336 = 1/12
yes: 15 over 37
a/7 + a/4 = 4a/28 + 7a/28 = 11a/28