Answer
$a=10.$
Work Step by Step
The cross product of parallel vectors is the zero vector.
${\bf (2i+4j-5k)}\times{\bf (-4i-8j+ak)}=\left|\begin{array}{rrr}
{\bf i} & {\bf j} & {\bf k}\\
2 & 4 & -5\\
-4 & -8 & a
\end{array}\right|$
$=(4a-40){\bf i}-(2a-20){\bf j}+(16-16){\bf k}$
For this to be the zero vector, it must be that $\left\{\begin{array}{l}
4a-40=0\\
2a-20=0
\end{array}\right.$
that is, $a=10.$
