Mandarin Equivalent: "duh!" "duh!" 

Advanced English Dictionary


  duh
  
  Disdainful indication that something is obvious. 
  
  "It’s hot in the desert. - Well, duh!"


Is there a Mandarin equivalent of 'duh!'?
Yes, there is: 


  废话


Example:

A: You know what, a banana without the skin is more tasteful!
B: 废话。


废话 is not a good word for kids to imitate, though. And it is rarely used between strangers or to your parents, as it may be impolite/offensive.