$1-6$ A revolving screen having a diameter of $1m$ and inclined at an angle of

$15$ to the horizontal is to be used to screen a particular material of

density $=2700k\frac{g}{m^3},$ at the rate of $5to\frac{n}{h}.$ The coefficient of friction

between the material and the floor of the drum is $0\mathrm{.}3.$ If the efficiency of

the installation is $90\%,$ calculate

$($a$)$ the maximum speed of revolution of the drum, $N$

$($b$)$ the depth of material in the drum

$($c$)$ the minimum power required to drive the drum