Question

Can someone also explain how to get the answer?

Answer 1

$\dfrac{(n-2)\cdot180}{n}\\\\ \dfrac{(18-2)\cdot180}{18}=16\cdot10=160$

Answer 2

Look at the picture

$\beta=360^o:18=20^o\\\\2\alpha+\beta=180^o\ (a\ triangle)\\\\2\alpha+20^o=180^o\ \ \ |subtract\ 20^o\ from\ both\ sides\\\\2\alpha=160^o\\\\2\alpha\ is\ an\ interior\ angle.\\\\Answer:\boxed{160^o}$