The mass of water that is in 150 g of Mgso4.7H20 is calculated as follows
mass of water= molar mass of H20 in MgSO4.7H20 /total molar mass of MgSO4.7H20 x mass ( 150g)
molar mass of h20 = 18 x 7= 126 g/mol since there is 7 water molecules
molar mass of MgSO4.7H20= 24 + 32 + (16x4) + (18x7) = 246g/mol
Mass of water is therefore= 126 g/mol/246g/mol x150g =76.8 g