The molar mass of urea is 60 g/mol.
1 mole of solution will contain 0.2 mole of urea and 0.8 mole of water. This corresponds to 12 g urea and 14.4 g water. Total mass of the solution will be 26.4 g.
Thus, 500 g of solution will contain 12×50026.4=227.27 g urea which is equal to 3.7878 moles.
The mass of water in diluted solution =1500−227.2=1272.72 g. This is equal to 1272.7218=70.70 moles.
The mole fraction of urea in the diluted solution is 3.787870.70+3.7878=0.05.