125 is a whole number and is, therefore, already in its simplest form.
If that was a typo and you meant to say 12/5, the answer is 2 2/5.
5 fits inside of 12 two times, with 2 left over.
We can check this by multiplying 5 by 2 (denominator by remainder), and adding 2 (the numerator). With this, we get 12/5, and we know our math is correct.
1/25 is already is simplest form.