Wir betrachten die Abbildung

${\displaystyle f\colon \{1,2,3,4,5\}\times \{1,2,3,4,5\}\longrightarrow \{2,3,4,5,6,7,8,9,10\},\,(x,y)\longmapsto x+y.}$

${\displaystyle {}z\in \{2,3,4,5,6,7,8,9,10\}}$

${\displaystyle {}f^{-1}(\{z\})}$

${\displaystyle {}f^{-1}(\{z\})}$

${\displaystyle {}\sum _{z\in \{2,\ldots ,10\}}{\#\left(f^{-1}(\{z\})\right)}}$