An infinite soil slope with an inclination of ${35}^{\circ }$ is subjected to seepage parallel to its surface. The soil has C' = $100kN/m^2$ and ${\varphi }^{\prime }={30}^{\circ }$. Using the concept of mobilized cohesion and friction, at a factor of safety of 1.5 with respect to shear strength, the mobilized friction angle is