Is it possible to calculate the propagation delay if only the length of the link is known & not the speed at which a bit travels into the link? Consider a $10^7$ bps link that is $400$ km long, with a queue large
  enough to hold $2000$ packets.Assume that the packets arrive at the
  queue with an average rate of $4000$ packets per second and that the
  average packet length is $2000$ bits.Find the average number of
  packets in the queue.


I am getting Average Queue length = 0.  



In this question is it possible to calculate Propagation delay?
How can we calculate the propagation delay until we don't know about the speed at which a bit travels in the link?
I believe your approach is right. Since we don't have any other information given. Then this means the packets are getting consumed at the rate same as link bandwidth. So Avg packet waiting must be 0.