There are two tennis courts. Pairs of players arrive at a rate of 3 per hour and play for an amount of time with mean 1 hour. A pair of players will wait for a mean amount of time of 30 minutes for their turn to play; otherwise, they will leave without playing. If there are already two pairs of players waiting new arrivals will leave. Assume that the number of pairs of players in the courts (both playing and waiting) at any given time form a birth and death process.Write down the corresponding Kolmogorov's backward equations.