[tex]\overbar{x3y}+\overbar{y7x}=504\\
\\ 100x+30+y+100y+70+x=504\\
\\ 101x+101y+100=504~|-100\\
\\ 101(x+y)=404~|:101\\
\\ x+y=4\\
\\ \\
\\ \overbar{xy}~e~un~nr~natural~de~doua~cifre \Rightarrow x~nu~poate~fi~0\\
\\ \\
\\ Cazul~1:~y=0 \Rightarrow x=4\\
\\ Cazul~2:~y=1 \Rightarrow x=3\\
\\ Cazul~3:~y=2 \Rightarrow x=2\\
\\ Cazul~4:~y=3 \Rightarrow x=1\\
\\ \\
\\ \overbar{xy} \in \{40,~31,~22,~13\}[/tex]
