The value of the angle of the incline 
at which the block starts to slide is the angle at which the component of the weight parallel to the incline becomes equal to the frictional force that keeps the block on the incline:
where the term on the left is the component of the weight parallel to the incline, and the term on the right is the frictional force, which is the product between the coefficient of friction
and the normal reaction of the incline N.
The normal reaction of the incline, N, is equal to the component of the weight perpendicular to the incline:
Therefore, the initial equation becomes
From which we find
For angles above this value, the block will start sliding down, otherwise the block will stay on the incline.
where the term on the left is the component of the weight parallel to the incline, and the term on the right is the frictional force, which is the product between the coefficient of friction
The normal reaction of the incline, N, is equal to the component of the weight perpendicular to the incline:
Therefore, the initial equation becomes
From which we find
For angles above this value, the block will start sliding down, otherwise the block will stay on the incline.