Show that any positive rational number can be expressed as $\frac{a^3+b^3}{c^3+d^3}$. [duplicate]

Question

Show that any positive rational number can be expressed as $$\frac{a^3+b^3}{c^3+d^3}$$

Perhaps the statement means that for every two positive integers $m$ and $n$, there exists a $k$ such that $mk$ and $nk$ both can be expressed as the sum of 2 cubes.

Personally, I have no idea how to solve this problem. Please help.

Thanks.