100 g solutie.................15 g .............85 g apa
150 grsolutie..................x g substanta.............yg apa
x=15x150/100=22.5 g substanta
y=127.5 g apa
daca se evapora 25 g apa raman 127.5-25=102.5 g apa
102.5+22.5 g subst=125 g solutie ramasa dupa evaporare
125 g solutie................22.5 g subst
100g solutie.................z g subst
z=2250/125=18
deci conc solutiei obtinute e 18%