A sequence of numbers is written in the following fashion: 1, 7, 1, 1, 7, 7, 1, 1, 1, 7, 7, 7, ... till ‘n’ terms. The sum of first 5001 terms of the given sequence is

Hi Peeyush,

If we take 1 and 7 as a set 

Number of terms  = 2+4+6......... ~5001

n(n+1) ~5001

70(71) = 4970

hence, in the end there will be 70 1's and 70 7's  and 31 1's after that

 Sum  = 31(1)+(4970/2)(8) =19911

Hope that helps.

Thanks,

Varun.