Nr de 3 cifre sunt :100.......999
x:57=c rest 31
x=57*c+31
daca c=2, x=57*2+31=114+31=145
c=3, x=57*3+31=171+31=212
........................................................
c=16, x=57*16+31=912+31=943
Sunt 15 nr
Suma lor este 57*2+31+57*3+31+......+57*16+31=31*15+57(2+3+...+16)=
465+57*(16+2)*15/2=465+57*9*15=465+7695=8160
