[tex]numerele\, de\, 3 \,cifre\, se\. incadreaza\,asfel:\,\,\,100\le\overline{abc}\le999/\\
100:31\approx3\,\,\si\,\,999:31\approx32\\
\Rightarrow\,\,\overline{abc}=\{3*31+13=106\,,\,4*31+13=137\,,\,...\,31*31+13=974\}\\
S=106+137+...+974=\\
=106+(106+1*31)+(106+2*31)+...+(106+28*31)=\\
=106*(28+1)+28*31=3074+868=3942[/tex]