molality is the number of moles of solute dissolved per 1000 g or 1 kg of solvent. then please tell me in the following question why we not take mass of solvent as mass of solution-mass of urea(m) i.e (0.25-m)

no of moles of urea = .25The mass of solvent = mass of solution - mass of solutemass of solution = 2.5kgmass of solute = no of moles x molecular mass                      = .25 x 60 =15 g  or 15 x 10-3 kgThe mass of solvent = 2.5 kg - 15 x 10-3                               =2.485 kgmolality = no of moles / wt of solvent in kg            = .25 / 2.485  = 1.006g