Question 1137607
General term of sequence,  {{{56+(n-1)*2}}};


78 is at index   {{{56+2(n-1)=78}}}
{{{2n-2=78-56}}}
{{{2n=78-56+2}}}
{{{n=12}}}


Sequence is 12 terms in length.


SUM:
{{{highlight_green((12/2)(56+78))}}}


{{{804}}}