What is the measure of one interior angle of a regular decagon (10 sides)?

The sum of the interior angles of a decagon is $(10-2)(180^\circ)=1440^\circ$. In a regular decagon all $10$ angles are equal, so each is $\frac{1440^\circ}{10}=144^\circ$, choice C. Check using exterior angles: each exterior angle is $\frac{360^\circ}{10}=36^\circ$, and $180^\circ-36^\circ=144^\circ$. Choice B ($140^\circ$) is the interior angle of a regular nonagon. Choice A ($120^\circ$) is the interior angle of a regular hexagon. Choice D ($160^\circ$) is the interior angle of a regular polygon with $18$ sides.