see you have 20 gram NaOH. and molar mass of it is = 40 gram. so no. of moles= 20/40 = 1/2mole. now volume of solvent= 1250ml and density= 0.8 g/ml. so mass of solvent= 0.8 x 1250= 1000 gram= 1 kg. so molality= mole of solute/ mass of solvent in kg. =1/2/1= 1/2 molal= 0.5 molal.