concentratia / 100 = masa dizolvata / masa solutiei
in solutia initiala avem md = c x ms : 100 = 25 x 150 : 100 = 37,5 grame
in solutia finala, creste masa solutiei cu 200 g
ms1 = 150 g + 200 g = 350 g
noua concentratie, c2 = md x 100 : ms2 = 37,5 x 100 : 350 = 10.71%