ms1=200g NaCl
c1=4%
c2=10%
c1=md1*100/ms1 => md1=c1*ms1/100=4*200/100=8 g
ms1=md1+mapă
masa apei nu se modifică
md adăugat=md a
md2=md a+ md1
ms2=md2+mapă
ms2=md a+md1+maapă
ms2=md a+ms1
c2=md2*100/ms2
c2=(md a+md1)*100/(md a+ms1) => c2(md a+ms1)=(md a+md1)*100 => c2*md a+c2*ms1=100*md a+100*md1 =>
100*md a-c2*md a=c2*ms1-100md1 =>
100*md a-10*md a=10*200-100*8 => 90 md a= 1200 => md a=1200/90=13,33 g
