The interior angles of a polygon are in A.P.if the smallest angle is 120

The interior angles of a polygon are in A.P.if the smallest angle is 120 degree and the common difference is 5 degree, then the no. of sides in the polygon is?

Smallest angle=120degrees
Common difference=5

A P is 120, 125, 130,��…

The sum of interior angles of a polygon= (n-2)180
Hence Sum of n terms of an A P = (n-2)180
n/2 {2.120+(n-1)5} = 180(n-2)
5n2 -125n +720 = 0
n2 -25n +144=0
n=9 or 16
Hence number of sides can be 9 or 16