Just divide 360 by 12.

Then, you look for the interior angle, which is 180 - \frac{360}{12}

Now that you have this, subtract this from 360 degrees.

Exterior angle = 360 - (180 - \frac{360}{12}) = 360 - 180 + \frac{360}{12} = 180 - \frac{360}{12}