Answer
a). $30.1^{\circ}$
b). $21.8^{\circ}$
Work Step by Step
Let $\theta$ be the inclination of the slope.
So, $mgsin\theta= coefficient\, of \,friction \times mgcos\theta$
$\theta=tan^{-1} (coefficient \,of\, friction)$
a) $\theta= tan^{-1} (coefficient \,of\,static\, friction)=tan^{-1}0.58=30.1^{\circ}$
b) $\theta= tan^{-1} (coefficient \,of\,kinetic\, friction)=tan^{-1}0.4=21.8^{\circ}$
(coefficient values as per table 4.1 pg 124)
