The possible pairs are (1,2),(1,3),(1,4),(1,5),(1,6),(1,7),(1,8).
(2,3),(2,4),(2,5),(2,6),(2,7),(2,8)
(3,4),(3,5),(3,6),(3,7),(3,8)
(4,5),(4,6),(4,7),(4,8)
(5,6)(5,7)(5,8)
(6,7),(6,8)
(7,8)
Therefore total sum =(1*7)+(2*6)+(3*5)+(4*4)+(5*3)+(6*2)+(7*1)
=84