(a) Relative density of a substance is the ratio of its density to that of water.
Mathematically, we have
Relative density = density of substance / density of water
(b) Volume of iron cube = length x length x length
= 1.5 x 1.5 x 1.5
= 3.375 cm^3
Density = 50 /3.375
= 14.82 g cm^-3
This is greater than the density of water (1 g cm^-3), thus the object would sink.