concentratia / 100 = masa dizolvata / masa solutiei
masa solutiei = masa dizolvatului + masa apei
c / 100 = md / ( m apa + m d)
30 / 100 = md / ( 100 + md)
100 md = 30 ( 100+ md)
100 md = 3000 + 30 md
70 md = 3000
md = 3000 : 70 = 42,85 grame
ms = 100 + 42,85 = 142,85
