An infinitely long slope is made up of a $c-\phi$ soil having the properties: cohesion $(c)=20$ kPa, and dry unit weight $(\gamma_d)=16 \: kN/m^3$. The angle of inclination and critical height of the slope are $40^{\circ}$ and $5$ m respectively. To maintain the limiting equilibrium, the angle of internal friction of the soil (in degrees) is _______