If the interior angles of a polygon are in $AP$. with common difference 5° and smallest angle is 120°, then the number of sides of the polygon is

Let there be $n$sides of the polygon. Then, the sum

of all interior angles is $(2n - 4)$ right angles.

$\begin{array}{l}\frac{n}{2}\{240+(n-1\left)5\right\}=(2n-4)90.\\ n^2-25n+144=0\Rightarrow n=9,16\end{array}$

But, for$n=16$, the largest angle $=120+(16-1)5={195}^0$which is impossible.

Hence,$n=9.$