concentratia / 100 = masa dizolvata / masa solutiei
in prima solutie, masa dizolvata = c x ms / 100 = 3 x 300 / 100 = 9 grame
in a doua solutie, md = 9 x 300 / 100 = 27 grame
amestecand cele doua solutii, masa solutiei devine 300 g + 300 g = 600 grame
masa dizolvata = 9 g + 27 g = 36 grame
c /100= md / m sol
c = 36 x 100 / 600 = 6 %