Question

how much salt must be added to 60 litres of a 20% solution of salt to increase it to a 40% solution of salt

Answer 1

solution already contains 20% of salt .This means that there is 60×20% =12 g salt in it. we need to make it a solution of 40% salt so 60×40% = 24 .But the solution already contain 12g of salt and we need to make it into 24 so we will subtract 12 from 24 . 24-12=12.
Answer=12g salt must be added