Quotient of two free abelian groups of the same rank is finite? Let $A,B$ be abelian groups such that $B\subseteq A$ and $A,B$ both are free of rank $n$. I want to show that $|A/B|$ is finite, or equivalently that $[A:B
]$ (the index of $B$ in $A$) is finite.
For example, if $A=\mathbb{Z}^n$ and $B=(2\mathbb{Z})^n$ we have that $[A:B]=2^n$ (correct me if I'm wrong).
 A: I mention a slightly different approach to Jyrki's, though essentially equivalent, using the theory of finitely generated torsion-free modules over a principal ideal domain (every finitely generated free Abelian group is such a module over the pid $\mathbb{Z}$). If $A$ is a finitely generated torsion-free module over a PID $R$, and $B$ is a submodule of $A,$ then there is a basis $\{a_{i}: 1 \leq i \leq m \}$ of $A$ and there are elements $d_{1},d_{2},\ldots ,d_{m}$ of $R$ such that $d_{i} | d_{i+1}$ in $R$ for each $i$ and 
$\{ d_{i}a_{i}: 1 \leq i \leq m \}$ is a generating set for $B$ (since we did not assume that $B$ had a basis of the same size as $A$, some of the $d_i$ may be $0,$ which is why I did not say it was a basis for $B).$ In the case $R = \mathbb{Z}$, this links with Jyrki's answer because the $d_{i}$'s are the invariant factors of the associated matrix, and $[A:B] = \prod_{i=1}^{m}d_{i}$ when $A$ and $B$ have the same rank. This theorem about $\mathbb{Z}$-modules leads to the Smith normal form for integral matrices. 
A: Let $A\cong B\cong \mathbb{Z}^n$, with $B\subseteq A$.  Then we have the short exact sequence 
$$ 0\rightarrow B\rightarrow A\rightarrow A/B\rightarrow 0.$$
Tensoring with $\mathbb{Q}$ shows $A/B$ has rank $0$, so is torsion.
A: There are several ways - for example induction on $n$ should work. We can also use the stacked basis theorem, if so desired.
But let's do the following. Select a basis ${\mathcal B}_A =\{a_1,a_2,\ldots,a_n\}$ of $A$.
Let us similarly select a basis ${\mathcal B}_B=\{b_i,b_2,\ldots, b_n\}$ of $B$. As $B\subseteq A$ we get a matrix
$M=(m_{ij})$, with integer entries, uniquely determined by
$$
b_i=\sum_j m_{ij}a_j.
$$
I want to show that the matrix $M$ is non-singular.
Let $V$ be the vector space over $\mathbf{Q}$ with basis ${\mathcal B}_A$. Then ${\mathcal B}_A$
and ${\mathcal B}_B$ are both linearly independent over $\mathbf{Z}$ in $V$, because they were so in $A$. Thus they are both vector space bases of $V$, and $M$ is the change of bases matrix that is then non-singular.
The inverse matrix $M^{-1}$ then has the property that $(\det M)M^{-1}$ also has integer entries (this follows immediately from Cramer's rule). This means that (working inside $V$ again) the basis elements $a_i$ have the property that $(\det M)a_i$ is a linear combination of vectors $b_j$ with integer coefficients. Therefore $(\det M)a_i\in B$. As this holds for all $a_i$, we have shown that
$$
(\det M)A\subseteq B.
$$
The claim follows from this: $A/B$ is a finitely generated abelian group with exponent that is a factor of $|\det M|$, so it must be finite.
A more careful study (using stacked bases theorem) reveals that in fact we have
$$
[A:B]=|\det M|.
$$
