Question

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Answer 1

as long as the opposite of older is younger

Answer 2

but if i say 'john is older than susan' , the negative is 'john is less than or equal to susan'

Answer 3

lilia222 you are correct

Answer 4

You can mess with the language all you want. Almost whatever you develop will eventually break down. I find it most complete and nearly bulletproof to leave the sentence alone and just put a tag line on the front.

P = George is older than Susan
Q = Kendra is older than Susan
~P = It is not the case that George is older than Susan
~Q = It is not the case that Kendra is older than Susan