Annie has $300$ grams of a solution with a sugar concentration of $15 \%$, Becky has $400$ grams of a solution with a sugar concentration of $25 \%$ and Carol has $300$ grams of a solution with a sugar concentration of $35 \%$. They mixed the solutions together. What is the concentration of the resulting sugar solution? 
Overseas Competition->Knowledge Point->Word Problem Modules->Concentration Problems->Basic Concentration Problems