concentratia / 100 = masa dizolvata / masa solutiei
solutia initiala are masa ms = 400 grame
se adauga 200 grame si masa solutiei devine ms1 = ms + 200g = 400+200 = 600 grame
concentratia c1 =20%
aflam masa dizolvata, care e aceeasi in ambele situatii
masa dizolvata = ms1 x c1 : 100 = 600 x 20 : 100 = 120 grame
in solutia initiala, c = md x 100 : 400 = 120 x 100 : 400 = 30%