And another problem:

Some amount NaCL separate in two equal parts. The first part is dissolved in water, therefore result 800 ml of solution D1 5% w/v. Find:

a. What is the initial (the original, the starting) mass of NaCL ?

b. In how much g of water must dissolve the second part of NaCL, therefore is shaped solution D2 with contests 10% w/w ?