13/21 can't be reduced.
The greatest common factor of 16 and 21 is 1. Therefore, 16/21 cannot be reduced and is already in its simplest form.
21/40 cannot be reduced.
25/21 is already reduced to its lowest terms
It can be reduced from 16/240 to 1/15
5/16 cannot be reduced further.
It cannot be reduced.
736 and 21 are comprime so the fraction cannot be reduced.
The fraction 19/21 cannot be reduced any more.
1/16 cannot be reduced. In fact, no unit fraction can be reduced.
No.
Yes, 21/3 = 7/1