I'm trying to understanding using the pumping lemma to prove that a language is not regular. I sort of understand how it works when the language describes strings with a particular form, like in this example, but what do you set w to if there isn't a particular form.

The particular problem I'm trying to prove isn't regular: The language is the set of strings that is composed of 60% or more As and Bs with the alphabet {A, B, C, D}

Because the form of the strings is indeterminate (i.e. it doesn't follow a pattern like a^i b^i, I'm not sure how to divide the strings into a w and x, y, z.