The character table of $G/Z(G)$ and $G/N$ given knowledge of the character table of $G$

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Suppose for a group $G$, of which we completely understand its conjugacy class structure and character table, is there a simple algorithm to deduce the character table of $G/Z(G)$? Or more generally, $G/N$ for $N \trianglelefteq G$?

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The answer is "yes". You can identify the columns of the character table of $G$, which correspond to elements in $N$. Then you take only those characters $\chi$, which have $\chi(1)=\chi(g)$ for every $g\in N$. This gives you some rectangle matrix with some duplicate columns. Just delete duplicate columns until you end up with a square matrix. This is the character table of $G/N$.