M=kV.
To find k we put in the given values M=0.250kg (this is how I read “is.250kg”) and V=0.5m³:
0.250=0.5k so k=0.250/0.5=0.50 and M=0.50V.
So, M=20 means V=20/0.5=40m³.
If “is.250kg” is meant to be 250kg then k=500 and M=500V so when M=20kg, V=20/500=0.04m³.
Note that k represents the density of the material in kg/m³. The density of water is about 1000kg/m³. Therefore, M=500V would be a more realistic interpretation for a metal of low density (floats on water).