Write your fraction in its most reduced form. For example: 40/[6√2] = 20/[3√2].
Multiply both the numerator and denominator of your fraction by the radical in the denominator. This eliminates the radical in the denominator. Because you are doing this to both sides of the fraction, this will not change the value of the fraction itself. For example:
20/[3√2] = 20/[3√2] * √2/√2 = [20√2]/(3*2)
Divide all other common terms out, leaving your fraction in its most reduced form. You will likely have a radical in the numerator at the end. For example:
[20√2]/(3*2) = [10√2]/3