WHY must a pentagon add up to 540° (in pictures)

Because there are now 3 triangles in a pentagon, it must add up to $3times180°=540°$

A pentagon can be divided into 3 triangles so it is just a case of multiplying 180 (The total angles of a triangle) by 3

The pentagon can be made from 3 triangles.

Example 1

An equilateral pentagon will have 5 equal internal angles and would be

$540^circ/5=108^circ$

Each internal angle is 108°.

A pentagon will have 5 equal internal angles

But the interior angles would still add up to 540°.