What mass of salt should be weighed and how many g (ml) of water should be measured in order

What mass of salt should be weighed and how many g (ml) of water should be measured in order to prepare 250 g by mass fraction of salt in a solution of 20%?

To find out how much substance is in the mass of a solution of a certain concentration, you need to multiply these two values and divide by 100%. In our case: (20% * 250) / 100% = 50 g. Water should be taken 250 – 50 = 200 g (or ml).