We have,
Density of solid, d = 5000 kg/m3
Weight in air, W = 0.5 kgf = 0.5g N
Density of water, q = 1000 kg/m3
Let, V be the volume of the solid.
So, W = Vdg
=> 0.5g = V × 5000 × g
=> V = 10-4 m-3
So, Buoyant force is, B = Vqg = 10-4 × 1000 × g = 0.1 g N = 0.1 kgf
Thus, apparent weight = W – B = 0.5 – 0.1 = 0.4 kgf