Answer
$5A=\begin{bmatrix}15&5\\20&10\end{bmatrix}$
No: $\det 5A\neq5\det A$.
Work Step by Step
$5A=\begin{bmatrix}5\cdot3&5\cdot1\\5\cdot4&5\cdot2\end{bmatrix}=\begin{bmatrix}15&5\\20&10\end{bmatrix}$
$\det 5A=15\cdot10-5\cdot20=150-100=50$
$5\det A=5\begin{vmatrix}3&1\\4&2\end{vmatrix}=5(6-4)=10$
