concentratia/100 = masa dizolvata/ masa solutie
pentru solutia initiala 25/100 = masa dizolvata/ 100
masa dizolvata = 25 x 100 / 100 = 25 grame
dupa ce adaugam apa, creste masa de solutie si, evident, scade concentratia
c /100 = 25/ (100+200) = 25 x 100 : 300 = 25/3 = 8,333 %