Matrix inside a matrix with aligning rows

Question

How is it possible to make such a construction in LaTeX?

Answer 1

It's simple with blkarray:

\documentclass[12pt]{report}
\usepackage{blkarray}
\usepackage{amsmath}
\renewcommand{\arraystretch}{1.4}
\begin{document}
\[
\begin{blockarray}{rrccc}
            & von: & N    & K   & E   \\
\begin{block}{rr(ccc)}
    N       &      & 0    & 0   & 0,4 \\
    nach: K & A=   & 0,75 & 0   & 0   \\
    E       &      & 0    & 0,8 & 0,8 \\
\end{block}
\end{blockarray}
 \]
\end{document}

Answer 2

With {NiceArray} of nicematrix.

\documentclass[12pt]{report}
\usepackage{nicematrix}

\begin{document}

\renewcommand{\arraystretch}{1.4}
\[
\begin{NiceArray}{rrccc}
                          & \text{von:\enskip} & N    & K   & E   \\
    N                     &             & 0    & 0   & 0.4 \\
    \text{nach:\enskip} K & A=          & 0.75 & 0   & 0   \\
    E                     &             & 0    & 0.8 & 0.8 \\
\CodeAfter 
  \SubMatrix({2-3}{4-5})
\end{NiceArray}
 \]    

\end{document}