documentclass[11pt,a4paper]{article}
usepackage{graphicx}
usepackage{tabularx}
usepackage[table,xcdraw]{xcolor}
usepackage[margin=2.5cm]{geometry}

%Tabelle breite gleich & zentriert
newcolumntype{C}{>{centeringarraybackslash}X}

begin{document}

begin{table}[h]
rowcolors  {2}{lightgray}{white}
resizebox{textwidth}{!}{%
begin{tabularx}{1textwidth}{l|C|C}
rowcolor{white}
Aufgabe & Fall 1 & Fall 2     \[1ex] cline{1-3}
Anlage von $K_0$ & multicolumn{2}{c}{5943,79}  \ [1ex]
Erwirtschafteter Ertrag & 5999,40 & 10.462,50 \ [1ex]
Anlage von Erträgen & 7.288,48 & 12.710,56 \ [1ex]
end{tabularx}%
}
end{table}

begin{table}[h]
rowcolors  {2}{lightgray}{white}
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begin{tabularx}{1textwidth}{C|C|C}
rowcolor{white}
x / y & z / a & b / c   \[1ex] cline{1-3}
100 & 200 & 300  \ [1ex]
312 & 469 & 623 \ [1ex]
432 & 862 & 527 \ [1ex]
end{tabularx}%
}
end{table}

end{document}