Also the + operator is typed with Num; from a Haskell repl:

  λ> :t (+)
  (+) :: Num a => a -> a -> a

In case of average we need /:

  λ> :t (/)
  (/) :: Fractional a => a -> a -> a

So our type is going to be constrained by Fractional instead of Num). So an implementation could be (from here [1]):

  import Data.List
  average :: (Real a, Fractional b) => [a] -> b
  average xs = realToFrac (sum xs) / genericLength xs

[1]: https://stackoverflow.com/questions/2376981