A mineral oil has a density of 0.875g/cm3. If you spread 0.75g of this oil over the surface of water in a dish with a diamerter of 21.6cm, how thick is the oil layer?

Divide the mass by the density to get volume. Divide the volume by the area of the dish.

\frac{g}{\frac{g}{\mathrm{cm}^3}} = g \cdot \frac{\mathrm{cm}^3}{g} = \mathrm{cm}^3

\frac{\mathrm{cm}^3}{\mathrm{cm}^2} = \mathrm{cm}

So the units work out, so it even stands a good chance of being right.