50+80= 53+47= ; +80=130;
47+ =100; 130=80+ ; 100= + ;
230﹣70= 190﹣90= ; 70+ =230;
190﹣ =90; ﹣160=70; ﹣90= +10.
50+80= 53+47= ; +80=130;
47+ =100; 130=80+ ; 100= + ;
230﹣70= 190﹣90= ; 70+ =230;
190﹣ =90; ﹣160=70; ﹣90= +10.