Sorry about the title - I wasn't sure about how to be more specific.
This is a homework problem, where I've only been able to write down (ii).
Let (S,*) be a binary system. Define the opposite operation *' to * by a*'b=b*a.
(i) Prove that if * is associative, then so is *'.
(ii) Give an example where (S, *) and (S, *') are not isomorphic. <--- Done.
(iii) Prove that if (S,*) is a group, then (S, *') and (S, *) are isomorphic.