Amy mixes $$30$$ g of a $$30\textbackslash\%$$ salt solution and $$20$$ g of a $$20\textbackslash\%$$ salt solution together. How many g of water should she add to the mixture to make it a $$10\textbackslash\% $$ solution? 
Overseas Competition->Knowledge Point->Word Problem Modules->Concentration Problems->Finding and Making Use of Invariants
$$30\times30\textbackslash\%+20\times 20\textbackslash\%=9+4=13$$ g.  $$13\div10\textbackslash\%-(30+20)=80$$ g.  ~~ 