Inside you pentagon, a triangle is drawn. This triangle is isosceles, because the corners are on the points of the regular pentagon. You know that angle C is 108°.
Angles in a triangle add up to 180°.
B and D will be the same size as this is isosceles.
180°-108° = 72°
This is B and D. You only need one of these, so you divide by 2.
