Ans: C
Since $x:y=8:5$, then let $x=8k$ and $y=5k$, where $k \neq 0$. Hence, we have
$\begin{array}{rcl}
2x & = & 4z -3y \\
2(8k) & = & 4z -3(5k) \\
4z & = & 31k \\
z & = & \dfrac{31k}{4}
\end{array}$
Therefore, we have
$\begin{array}{rcl}
y :z & = & 5k : \dfrac{31k}{4} \\
y:z & = & 20 :31
\end{array}$
