Amy mixes $$30$$ grams of a $$30\textbackslash\%$$ salt solution and $$20$$ grams of a $$20\textbackslash\%$$ salt solution together. How many grams 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
$$30\times30\textbackslash\%+20\times 20\textbackslash\%=9+4=13$$ ounces.  $$13\div10\textbackslash\%-(30+20)=80$$ ounces. 