Probability part of discrete math course

(e) A country has exactly 10 major cities. The government is adding flights between pairs of cities. Suppose the funding allows for exactly 18 bidirectional flights to be added. What is the probability that city A and city B are connected by a flight? What is the probability that one can flight city A to city B to city C? (each a direct flight). What is the expected number of flights out of city A? Suppose now that exactly 5 of those cities apply a tax at their airport. What is the probability that there are exactly 6 flights for which the tax is applied