A contractor has to decide how much to bid for a construction project. It will cost $4,000 to prepare the bid. This cost would be incurred whether or not the bid was accepted. The previous winning bids for similar projects have been (in $1000s): 80, 60, 70, 60, 60, 80, 80, 60, 80, 80, 50, 50, 60, 60, 70, 70, 50, 80, 60, 70, 50, 80, 70, 60, 80, 80, 80, 90.

If the contractor wins the bid, the cost of completing the project will be $50,000. Construct a decision tree and a simulation to determine the optimum bid.