A block rests on a board. The coefficient of static friction between them is 0.55.

On a slope at angle $\theta$, split the weight: $mg\sin\theta$ down the slope and $mg\cos\theta$ into it, so $N = mg\cos\theta$. Sliding down a rough slope, friction $\mu_k mg\cos\theta$ acts up it:

A body at rest starts to slide when $\tan\theta > \mu_s$.