At critical speed, the top ball’s centripetal acceleration equals gravity: $\omega^2(D-d)/2=g$. Thus $N_c=(60/(2\pi))\sqrt{2g/(D-d)}$. Substitute diameters in metres: $N_c=(60/6.28)\sqrt{19.62/(3.60-.16)}\approx22.82$ rpm. Rounded to an integer,**23 rpm**.
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