A triangle has 180° and a quadrilateral (4 sides) has 360°. For each additional side we add 180° so a 5 sided figured has 360 + 180 = 540° total. 540/5 (5 sides) = 108° per angle.
Formula for any number of sides where s is the number of sides = (s-2)180°
Let's try if for 3 sides (3-2) 180 = 1 (180) = 180°
Now four sides (4-2) 180 = 2(180) = 360°
And finally, your pentagon = (5-2) 180 = 3(180) = 540°
and 540 shared equally in a regular pentagon (regular means all sides and angles are the same) means 108 degrees in each angle 540÷5 = 108°
