Have a running total of 3 x 15 minute blocks - take this figure away from 78 and this leaves the number for the 4th quarter hour. Count the expected number of arrivals and then this leaves the number of departures. So 78 - (22 + 19 + 17) = 20, If there were 12 expected arrivals for the 4th quarter hour then 8 departures would be permitted. Doesn't include State or Hospital (or Medevac flights) but does include helicopters landing or taking off from a runway.