Buoyant forces with 2 liquids question

[crispy_nine]

Hi, I have a tricky question, and was hoping somebody could explain to me how I would go about solving it. Here it is:
Oil having a density of 930 kg/m3
floats on water. A rectangular block
of wood 4.00 cm high and with a
density of 960 kg/m3 floats partly in
the oil and partly in the water. The
oil completely covers the block. How
far below the interface between the
two liquids is the bottom of the
block?
the answer is x = 1.71 cm
Any help is much appreciated.

 

[Doc Al]

What have you done so far? Start by finding the forces acting on the top and bottom of the floating block.

 

[arildno]

It can also be advantegous to determine the weight of the block.