$$\dfrac{100\times50\textbackslash\%+400\times80\textbackslash\%}{100+400}=74\textbackslash\%$$. 
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. 
Options:
  $$70\textbackslash\%$$ 
  $$72\textbackslash\%$$ 
  $$75\textbackslash\%$$ 
  $$74\textbackslash\%$$ 