A rectangular container, of base dimensions $0.4 \mathrm{m} \times$ $0.2 \mathrm{m}$ and height $0.4 \mathrm{m},$ is filled with water to a depth of $0.2 \mathrm{m} ;$ the mass of the empty container is $10 \mathrm{kg} .$ The container is placed on a plane inclined at $30^{\circ}$ to the horizontal. If the coefficient of sliding friction between the container and the plane is $0.3,$ determine the angle of the water surface relative to the horizontal.