[tex]\bf \textit{28 consecutive numbers}\to
\begin{cases}
a\\
a+1\\
a+2\\
a+3\\
a+4\\
a+5\\
a+6\\
a+7\\
a+8\\
a+9\\
a+10\\
a+11\\
a+12\\
a+13\\
a+14\\
a+15\\
a+16\\
a+17\\
a+18\\
a+19\\
a+20\\
a+21\\
a+22\\
a+23\\
a+24\\
a+25\\
a+26\\
a+27
\end{cases}[/tex]
now, their sum is 686
meaning
(a)+(a+1)+(a+2)+(a+3)+(a+4)+(a+5)+(a+6)+(a+7)+(a+8)+(a+9)+(a+10)+(a+11)+(a+12)+(a+13)+(a+14)+(a+15)+(a+16)+(a+17)+(a+18)+(a+19)+(a+20)+(a+21)+(a+22)+(a+23)+(a+24)+(a+25)+(a+26)+(a+27) = 686
solve for "a"
