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%I #6 Apr 24 2020 18:41:57
%S 5,54,549,5499,54999,549999,5499999,54999999,549999999,5499999999,
%T 54999999999,549999999999,5499999999999,54999999999999,
%U 549999999999999,5499999999999999,54999999999999999,549999999999999999,5499999999999999999
%N Arithmetic mean of all n-digit positive even numbers.
%F a(n) =1/2{10^(n-1) + 10^n -2}
%e a(2) = Arithmetic mean of 10,12,14,...96,98 =(10 + 98)/2 =54.
%t Table[FromDigits[PadRight[{5,4},n,9]],{n,20}] (* _Harvey P. Dale_, Apr 24 2020 *)
%K base,easy,nonn
%O 0,1
%A _Amarnath Murthy_, Aug 02 2005
%E Corrected and extended by _Joshua Zucker_, May 10 2006