When the magnet is placed on the glass, it is attracted to the iron filings. The pattern of the iron filings shows that the lines of force that make up the magnetic field of the magnet. Also, The lines of force of north and south poles attract each other whereas those of two north poles fend off each other.
For the position of button we can use force balance as
Friction Force = Centripetal force
so here we will have

here we know that
R = radius of circle where button is placed

f = 35 rev/min


now from above equation


so friction coefficient will be 0.33