I need to solve this problem using conditional expectation:

A hen lays N eggs where N is Poisson with parameter k. The weight of the nth egg is Wn where W1, W2, ... are independent identically distributed random variables with mean x. Let W=sum from i=1 to i=N of Wi.

Show E[W]=xk

Any help would be hugely grateful! (Smile)

p.s. sorry for my poor inputting, im a newbie!