I'm going to assume this is (4m / 5) / (12m^2 / 15)
Turn (12m^2 / 15) upside down and multiply by the numerator.
(15 / 12m^2) (4m / 5)
Reduce the 15 and 5 by dividing top and bottom by 5
Reduce the 12 and 4 by dividing top and bottom by 4
Reduce m^2 and m by dividing top and bottom by m
3/3m
Reduce the 3 and 3 by dividing top and bottom by 3
1 / m is the answer.