Find the number of all three elements subsets of the set`{a_1, a_2, a_3, ........... a_n}` which contain `a_3dot`

The number of all three element subsets of the set {a_(1),a_(2),a_(3)......a_(n)} which contain a_(3) , is

FOr an integer nlt2, we have real numbers a_1,a_2,a_3,a_4,..........,a_n such that a_2+a_3+a_4+.......+a_n=a1 and a_1+a_3+a_4+...........+a_n=a_2...............a_1+a_2+a_3+..............+a_(n-1)=a_n what is the value of a_1+a_2+a_3+........+a_n?

The number of all subsets of a set containing 2n+1 elements which contains more than n elements is

If a_(n) = n(n!) , then what is a_1 +a_2 +a_3 +......+ a_(10) equal to ?