When 400 g of a 20% solution by weight was cooled, 50 g of solute precipitated. The percentage concentration of remaining solution is

weight of solution = 400g

weight of solute = 20 x 400/100 = 80 g

weight of solute precipitated = 50g

weight of solute remained in solution = 80-50 = 30g

weight of solution remained = 400 – 50 = 350g

Hence % by weight = 30/350x 100 = 8.57%