Hi, Are you saying the we don't know the actual number of c and d so that we can't know the average of women salaries? Is this your answer?
You've plugged in a set of numbers to get one set of answers. What if the number of men and women is opposite of what you've assumed, in which case we'll get a different answer. To know a definitive answer for average, we need the total salary and the number of employees (i.e. c, d etc.). With the given information, we can't deduce about the average Salaries (or to be more precise, the relative values of average salaries, as the question asks) of men/women
I think plugging in numbers is effective mostly for eliminating answer choices, and not for arriving at an answer choice with absolute certainty.