Move the decimal points 3 places to the right in both the numerator and denominator so you only have integers. 12.5 becomes 12500 while .025 becomes 25.
12500
---------
25
Now it is a simple division:
........ 500
25) 12500
The usual way is to move the decimal point enough places to mean you can ignore them.
12.5/.025 is the same as 125/.25 (decimal place has been moved one place),
which is the same as 1250/2.5 (d.p. has moved 2 places) and that's the same as
12500/25 so now you don't need a calculator (maybe).
4 lots of 25 make 100 so 500 lots of 25 make 12500.
Sometimes, the decimal number allows a few shortcuts. For example this one (after 1 move of the point) became 0.25 or 1/4. That's a nice easy fraction. I know 4 of them make 1 so 500 of them make 125.
You have to multiply both numbers with 1000 (that is because tge divisor has 3 digits after decimal point, and you have to divide without decimal poin t, if you understand what I mean)
So you will have
12500/25=2500/5=500
I hope you understand - my English is not good enough to explain it better!
anyway - good luck!