Explanation Suppose the angles are: a, a+d, a+2d, a+3d, a+4d, a+5d, a+6d, a+7d
Given that: (a+6d + a+7d)/2 = 153^{0}
=> 2a + 13d = 306 -------- (1)
We know that sum of all interior angles of a polygon of n sides is: (n-2)*180
So, a + a+d + a+2d + a+3d + a+4d + a+5d + a+6d + a+7d = (8-2)*180
=> 8a + 28d = 1080
=> 2a + 7d = 270 -------- (2)
(1) - (2) gives: 6d = 36
so, d = 6
Solving from (1) we get a = 114
So, the smallest = 114
second smallest angle = 114 + 6 = 120
So average of first two smallest is: (114 + 120)/2 = 117