let

The sum will be sum of (number of three sets with i as the minimum element multiplied by i)

so 1 x ⁵C₂ + 2⁴C₂+3³C₂+4²C₂

* X should be the set of subsets of A containing 3 elements..

I toook n = 6 (Dont know why)
so take the sum accordingly...

Σ r ^n-rC₂ (limit is for r going from 1 to n-2)

There should be some more elegant way because of the form of the answer.. but I cant think of the more "elegant method" to get N+1C4

Btw you even need to verify that the expression I have given is equal to that ..

let r be the lowest value ..

for each r, there will be ^n-rC₂ sets which will have r as the smallest elemetn.. because we have to chose 2 from the elements larger than r