# Math Help - Groups and Subgroups #2

1. ## Groups and Subgroups #2

Suppose H is a subgroup of the finite group G. Show that there exists a subset {z1,...zr} of G which is simultaneously a set of representatives of the left and of the right cosets of H in G, that is, G is a disjoint union of the ziH and also of the Hzi.
I'll appreciate you help.