Amy mixes $$10$$ g of a $$20\textbackslash\%$$ sugar solution and $$40$$ g of a $$25\textbackslash\%$$ sugar solution together. After~\uline{~~~~~~~~~~}~of water evaporates, the percent concentration of solution is $40\textbackslash\%$. 
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$$10\times20\textbackslash\%+40\times 25\textbackslash\%=12$$ g.  $$(10+40)-12\div40\textbackslash\%=20$$ g. 