ρWater = 1g/cm^3
1) For ice cubes: m = ρIce * VIce, VIce = m/ρIce.
Fused to water, V water = m/ρ water.
V ice - V water = m/ρ ice - m/ρ water = 0.5*10cm^3;
i.e.: m(10/9-1)=5;
So: m=45g.
V ice = m/ρ ice = 50cm^3
2) Mass of the stone M=55-m=10g
3) Ice and stone are suspended in the water, the total volume V0=m/ρwater=55cm^3,
Volume of the stone: V stone=V0-V ice=5cm^3.
That is, ρ stone=M stone/V stone=10/5=2g/cm^3, i.e.: 2*10 cubic kg/cubic meter.