A sample of 50 litres of glycerine is found to be adulterated to the extent of 20%. How much pure glycerine should be added to it so as to bring down the percentage of impurity to 5% ?

Answer: Option B

Give the amount of solution = 50litres
20% of impurity in a given solution
i.e.
40 liters of glycerine and 10 liters of impurities
Now to keep the impurity 10 liters and we added glycerine to a solution to bring down impurities level 5%.
i.e. 10 is 5% of 200. So we need to added glycerine 'x' amount of glycerine to make solution of 200liters
∴ x = 200 - (40 + 10) = 150 litres of glycerine