Question

What is the sum of the degrees of the interior angles of a decagon?

Answer 1

The measure of the angle of a regular decagon is 144 degrees.

Answer 2

But the sum of the interior angles of a decagon is 1440 degrees.

Answer 3

That makes it 1440 :)
A triangle can fit one triangle in it, so its angle sum is 180
A quadrilateral can be cut into two triangles, so its angle sum is 360
A pentagon can be cut into three triangles, so its angle sum is 540

See the pattern?
For any n-gon, the sum of the angles would be
180(n - 2), where n is the number of sides