Question 1100176
{{{4+5x=104}}}

{{{5x=100}}}

{{{x=20}}}


The sequence has 21 terms.  Middle term is at index 11.
{{{4+(11-1)5}}}
{{{4+10*5}}}
{{{highlight(54)}}}



Also, {{{(4+104)/2=108/2=54}}}, average of first and last terms of sequence.