Average Out Two Lists
Pyth, 24 bytes

?}KcsJsQlJmcsdldtPyJK":(


Try it online: Demonstration

Thanks to Dennis for noticing an error and golfing one byte. 

Explanation:

?}KcsJsQlJmcsdldtPyJK":(   implicit: Q = evaluated input
      sQ                   all numbers of Q
     J                     save them in J
  KcsJ  lJ                 average of J (sum(J) / len(J))
                           store in K
          m     tPyJ       map each nonempty subset d of J to:
           csdld             average of d
?}                         if K in ^:
                    K        print K
                     ":(   else print sad-face