All finite abelian groups are of the form (Z_a)x(Z_b)x...x(Z_n) and anything of this form is a finite abelian group. So if your group is of order 20, it must be one of (Z_20), (Z_10)x(Z_2), (Z_5)x(Z_4), (Z_5)x(Z_2)x(Z_2). But (Z_mn)=(Z_M)x(Z_n) iff m,n are
coprime. So (Z_20) is isomorphic to (Z_5)x(Z_4). (Z_4) is not iso to (Z_2)x(Z_2) because 2 and 2 are not coprime. Also, (Z_20) is not ...