Source: Rebecca Gowers. Plain Words (2014 ed). p. 186 Top. I numbered all numerals.
Avoid multiple negatives when you can. Even if you dodge the traps they set and succeed in saying what you mean, you give your reader a puzzle to solve in sorting the negatives out. Indeed it is wise never to make a statement negatively if it could just as well be made positively. A correspondent sends me this:
[1.] The elementary ideas of the calculus are not beyond the capacity of
more than40% of our certificate students,
He comments, "I am quite unable to say whether this assertion is that 2/5 or 3/5 of the class could make something of the ideas". If the writer had said that the ideas were within the capacity of at least 60%, all would have been clear.
If I omit modifiers in 1, then 1 becomes 2:
2. The ideas are not beyond the capacity of [some unknown percent] > 40%.
3. Am I correct that not beyond = within? Then 2 becomes:
4. The ideas are within the capacity of [some unknown percent] > 40%.
5. Is 4 correct? If yes, what's ambiguous? Why's the correspondent flummoxed?