Probability — practice question

A uniform solid cube with edges of length $0.4\,\text{m}$ is in equilibrium on a rough plane that is inclined at $30\degree$ to the horizontal. $ABCD$ shows a cross-section through the cube’s centre of mass, with $AB$ lying along a line of greatest slope. $B$ lies below $A$. One end of a light elastic string, of modulus of elasticity $12\,\text{N}$ and natural length $0.4\,\text{m}$, is fixed to $C$. The other end is fixed to a point on the same line of greatest slope below $B$, so that the string makes an angle of $30\degree$ with the plane (see diagram). The cube is about to topple.