Jack mixed $200$ g of $10\textbackslash\%$ salt solution, $200$ g of $15\textbackslash\%$ salt solution, and $400$ g of $20\textbackslash\%$ salt solution together. What is the percent concentration of salt solution at this time? 
Overseas Competition->Knowledge Point->Word Problem Modules->Concentration Problems->Basic Concentration Problems->Calculating Solution from Solute and Solvent
$$\frac{200\times 10\textbackslash\%+200\times 15\textbackslash\%+400\times 20\textbackslash\%}{200+200+400}\times 100\textbackslash\%=16.25\textbackslash\%$$. 