If a regular polygon has 40 sides the measure of each interior angle will be

The formula for interior angles is:



Using this, plug in 40 as n and find s.
If you do that, you will get s=6840.
Now that you have the sum of interior angles, you have to find one single measure of an angle in your regular polygon. 
If 40 equal sides in a regular polygon results to 6,840°, then one vertex should be the sum of the interior angles divided by the total number of sides.

In other words, you will have to find 6840°/40. You will get 171°.  That means that the measure of a single interior angle is 171*.