Diameter of a circle is 16cm what is the radius?

The diameter of a circle is the length of a straight line from a point on the edge of the circle passing through the center of the circle and onto a point on the opposite edge of the circle.

The radius of a circle is the length of a straight line from the center of a circle to a point on the edge of the edge of the circle.

So the radius of a circle is half its diameter, in this case 8cm.