Answer: All people do like walking
It might be more grammatically correct to say "all people like walking", but I wanted to include the "do" in there because we can see how the "don't" flips to "do". The "some" flips to "all" as well.
The negation of a statement is the complete opposite in every way. If a friend makes a claim that some people don't like walking, and you can prove that all people do like walking, then this contradicts your friend. The same can be said in reverse: if your friend claims "all people do like walking", then you can counter back with "some people don't like walking". All it takes is one counter-example to prove the claim "some people don't like walking" to be correct. In terms of logic, the word "some" means "one or more".
