Irene mixes $$100$$ grams of dogfood that contains $$50\textbackslash\%$$ rice with $$400$$ grams of dogfood that contains $$80\textbackslash\%$$ rice. Find the percent concentration of the rice in the new mixture. 
Overseas Competition->Knowledge Point->Word Problem Modules->Concentration Problems->Basic Concentration Problems
$$\dfrac{100\times50\textbackslash\%+400\times80\textbackslash\%}{100+400}=74\textbackslash\%$$. 