[tex]100\le\;M_{40}+17\le999\\
\Downarrow\\
100:40=2,5\approx3\;\;\;si\;\;999:40=24,975\approx24\\
M_{40}+17\in\,\{3*40+17=137;4*40+17=177\;...\;24*40+17=977\}\\
numarul\;numerelor\;de\;3\;cifre\;conform\;cerintei\;sunt:\\
24-3+1=\fbox{22}[/tex]
nr. sunt ... 3*40+17=137;4*40+17=177;5*40+17=217 ; ... ;23*40+17=937;24*40+17=977
S = 22*17+40*[+(1+2)+3+4+5+...+23+24 -(1+2)]=374+40*[24*25:2 - (3)]=
de aici cred ca te descurci !
