0,(1x)= (10+x)/99
0,(2x)=(20+x)/99
0,(3x)=(30+x)/99
...........................
0,(9x)=(90+x)/99
(10+x)/99+ (20+x)/99 + (30+x)/99 +....+(90+x)/99=x aducem la acelasi numitor
10+x+20+x+30+x+...+90+x=99x
9x+(10+20+30+...+90)=99x
9x+ (10+90)*9/2= 99x
9x +100*9/2=99x
9x +50*9=99x
9x +450 =99x
9x -99x= -450
-90x = -450
90x=450
x=5
