Question

. 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 1

Double the amount by adding pure glycerin and you reduce the percentage of adulteration to half 

you have to bring down the adulteration level from 20%-10% and then 10%-5% 
add more 50L glycerin so you have 100L solution with 10% adulteration 
add more 100L glycerin so you have 200L solution with 5% adulteration 

so intotal you add 150L of glycirine