Question 485643
add first and last

{{{1 = 500 = 501}}}

Do the same with the second highest and the second smallest.

{{{2 + 499 = 501}}}

As you can see, all the totals equal {{{501}}}. Now just divide {{{500}}} by {{{2}}} since you are using two unique numbers per combination.

{{{500/2 = 250}}}

So, the average is {{{(1+500)/2 }}}and there are {{{500}}} number is the list, so the {{{sum}}} is:

{{{(501/2)500 = 250*501=125250}}}