A regular pentagon ABCDE is shown

Work out the size of X

The sum of the angles in a pentagon is 540°.

This means that each angle is 108°.

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.

The answer is 36°.

Hope this helps :)