grammaticality - When do multiple negatives cancel and when do they not?

Which of the following is not stated in the passage?

(a) Money will not be a factor in making the decision

Here, not only is there a negative in the question, but there is also one in answer (a). It is important not to miss the second one, and also important not to simply think that they cancel each other out. It does not, for example, follow from (a) that what is being said is that ‘money is stated in the passage to be a factor in making the decision’.

I apprehend that silence isn't guilt (in most developed nations), so the absence of (a) in the passage doesn't mean the truth of (a). Yet what's the big picture here; what are the general lessons to be learned? I'd like to cancel, to simplify reading whenever possible.

Source: p 27, Mastering the National Admissions Test for Law, Mark Shepherd