When you have a powered number at the denominator of a fraction, you can multiply that power into a negative and put that powered number at the numerator.
An example:
500/(8 x10^-9) = 500 x 10^9/8
so:
25V / 50 x 10^-3 = 25V x 10^3/50
You could do this too:
25V/0.05= 500
or
25000/50= 500 (I guess you are able to see what I have done here according to the first paragraph).
So you see that the calculation of those kinds of numbers is so easy. So please do several exercise to be clever regarding to them