Sum of the 15 two elements subset of $\{1,2,3,4,5,6} = ( N-1) (N)(N+1)/2
Three elements....two at time (1,2) (1,3) (2,3)....sum = 12
Four elements .... two at a time (1,2) (1,3) (1,4) (2,3) (2,4) (3,4) .....sum = 30
Five elements....two at a time (1,2) (1,3) (1,4) (1,5) (2,3) (2,4) (2,5) (3,4) (3,5) (4,5)
Note that for N elements taken 2 at a time....the pattern of sums seems to be
= (N-1)(N)(N+1) / 2
For taking 6 elements taking 2 at a time the sum should be
=(5)(6)(7) /2
=210 /2
=105
Therefore, total sum of those 15 sums is = 105
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