Understanding Group Size Change: G/N in Abstract Algebra

[Leb]

This is not really a homework questions, rather a concept based one. I am studying from Fraleigh's ''Intro to abstract algebra'' and in chapter 15 it states, that for a group G and normal non-trivial subgroup of N of G, the factor group G/N will be smaller than G. I am not sure how he counts the change in size of the group.

For instance(ex from wiki), if we take G=Z₆ and it's normal subgroup N={0,3} we get G/N to be { {0, 3}, {1, 4}, {2, 5} } (i.e. all the cosets, which partition the whole of G). Or do we take each coset as a different element in G/N ?
That is a={0,3}, b= {1,4}, c={2,5} so that |G/N| = 3 ?

 

[HallsofIvy]

Yes, what you say is true. The elements of a factor group of group G are subsets of G. Further, they "partition" G- each element of G is in one and only one of these subsets. Finally, every coset contains the same number of elements. That is, if |G|= n and |N|= m then |G/N|= n/m.

If m= 1, then N is just the identity and G/N= G. In all other cases, m> 1 so n/m< n.