A disk like the lower one in Fig. 10-11 has a moment of inertia I = 0.015

Question:

A disk like the lower one in Fig. 10-11 has a moment of inertia I = 0.015 0 kg · m², and is turning at 3.0 rev/s. A trickle of sand falls onto the revolving disk at a distance of 20 cm from the axis and builds a 20-cm radius narrow ring of sand on it. How much sand must fall on the disk for it to slow to 2.0 rev/s? 1 + 700 Fig. 10-11