Calculate the mass of sodium sulphate required to prepare 20% solution in 100g of water

Dear student
The mass by mass percentage of a solution is given by the following relation

mass by mass percentage = (mass of solute / mass of solution) X 100

where 

mass of the solution = mass of solute + mass of solvent

Here we are given that the mass percent of sodium sulphate solution is 20 %. Let the mass of sodium sulphate which is required to prepare this solution be 'a' grams. We are given that the mass of water is 100 g. So

20 = [a / (a + 100)] X 100

Solving for a, we get 

a = 25g

Thus, 25g of sodium sulphate is required. 

Hope it is clear
Regards