A disk of 7.0 cm diameter emits light uniformly from the surface. 20 cm from this disk, all along its axis, is a 9.0 cm diameter opaque black disk; the faces of the two disks are parallel. 20 cm beyond the black disk is a white presentation screen. The lighted disk illuminates the screen; however there is a shadow in the center due to the black disk.
Determine the diameter of the fully dark part of this shadow?
Represent your answer to two significant figures and comprise the suitable units.