Forces and equilibrium — practice question

$ABC$ is a vertical cross-section of a surface. The section of the surface that contains $AB$ is smooth, and $A$ lies $4\,\text{m}$ above $B$. The section of the surface that contains $BC$ is horizontal, and $BC$ has length $5\,\text{m}$ (see diagram). A particle of mass $0.8\,\text{kg}$ is released from rest at $A$ and moves along $ABC$. Find the speed of the particle at $C$ in each of the following cases.