Hey Dimitri!
15 = 3*5
So, there will exist elements of orders 3 and 5 say 'a' and 'b'. that's correct. But then how can u conclude that there will exist an element in the group of order 15.
If group were abelian then this fact was true, bcoz then o(ab)= lcm (3,5)=15.
But here we have...
Hey!
We know that if there exists an element of a given order in a group, there also exists a cyclic subgroup of that order. What about converse?
Suppose there is a subgroup of an Abelian group of order 'm'. Does that imply there also exists an element of order 'm' in the Group. It does not...