A penny of mass 3.1 g rests on a small 26.2 g block supported by a spinning disk of radius 15 cm. The coefficients of friction between block and disk are 0.715 (static) and 0.64 (kinetic) while those for the penny and block are 0.511 (static) and 0.45 (kinetic). What is the maximum speed of the disk without the block and penny sliding? The acceleration of gravity is 9.8 m/s2 . Answer in units of rpm.