Jack mixed $20$ grams of sugar with $30$ grams of water. What is the percent concentration of the sugar solution? Then Jack added $50$ grams of water. What is the percent concentration the sugar solution? 
Overseas Competition->Knowledge Point->Word Problem Modules->Concentration Problems->Basic Concentration Problems->Calculating Solvent from Solution and Solute
$n\_1=\frac{20}{20+30}\times100\textbackslash\%=40\textbackslash\%$;  $n\_2=\frac{20}{20+30+50}\times100\textbackslash\%=20\textbackslash\%$. 