What is the size of each interior angle of a regular pentagon?

The answer is 108'. There is a nice way of answering this for any regular polygon (like a square, a pentagon, a hexagon etc): first split your regular polygon into triangles by joining one vertex (point where 2 lines meet) to all other vertices. You will be left with several triangles, (eg in a pentagon 3 triangles). The interior angles of a triangle add to 180', so we can see that the interior angles of our pentagon add to 3 x 180 = 540'. And there are 5 vertices so 540/5=108', which is out answer.