A solid disk of radius R and mass 2.66 kg is spinning at angular velocity 66.2 rpm about a vertical axis. A solid disk of radius 2R drops concentrically on the first (sort of like dropping an old-fashioned 33 rpm record on a 45 rpm one), so that eventually the two cease to slip relative to each other and end up spinning at 18.7 rpm. What is the mass of the second disk?
kg