Answer and Step-by-step explanation:
Every group homomorphism ϕ:Zn→Zm extends in a unique way to a linear transformation T:Qn→Qm of vector spaces over Q.
Moreover, ϕ is injective iff T is injective. But T being injective implies n≤m.
Applying this to both ϕ and ϕ−1, we conclude that n≤m≤n.
NOTE:
The proof and step-by-step explanation is in the attachment below
