Let, eight successive numbers is $X , (X+1) , (X+2) , (X+3) , (X+4) , (X+5) , (X+6) , (X+7)$
According to question, ${X+(X+1)+(X+2)+(X+3)+(X+4)+(X+5)+(X+6)+(X+7)}/8 =6.5$
$=> 8x+28 =52$
$=> 8x =24$
$=> X = 3$
Now, eight successive numbers is $3,4,5,6,7,8,9,10$
Smallest number $= 3$
Greatest number $=10$
The average of smallest and greatest number is $(10+3)/2 = 6.5$