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? 
选项为:A:$$70$$  B:$$72$$  C:$$75$$  D:$$80$$  请回答其中一个选项.