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I want to find the density of a rock - Fluid Dynamics Question

We suspend the rock with a string from a weighing scale. In the air, the weighing scale reads a mass of m = 0.22 kg. Then we completely submerge the rock in a glass of oil while it is still suspended from the weighing scale: the weighing scale now reads a mass of m′ = 0.01 kg. We know that the density of oil is ρ = 850 kg/m3. What is the density ρ_0 of the rock? You may neglect the buoyancy effect in air.
I am actually a maths student but have to study this fluid dynamics module which unfortunately I am terrible at. I have been sat for ages trying to do this question. I have the answer which is ρ_0=890 kg/m^3 but I don't know how to get there. I've tried playing around with Mass = Volume x Density etc ... but can't make it work. Would really appreciate a helping hand :smile: