Answer
$14^{\circ}$
Work Step by Step
We can find the required angle as follows:
$\Sigma F_y=N-mgcos\theta=0$
$\implies N=mgcos\theta$
Similarly $\Sigma F_x=mgsin\theta-f_s=0$
$\implies mgsin\theta-\mu_s (mgcos\theta)=0$
$\implies \mu_scos\theta=sin\theta$
This simplifies to:
$\theta=tan^{-1}(\mu_s)$
We plug in the known values to obtain:
$\theta=tan^{-1}(0.25)$
$\theta=14^{\circ}$
