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Actually you must also allow for the fact that the climb starts at 1000 feet over the aerodrome. So the climb is from 2250 feet to 7500 feet = 5250 feet. You must climb through 5250 feet in the same time it takes to travel 36 nm at 90 kt = 24 min. 5250 feet in 24 minutes = 219 ft/min. The QNH of 1023 is not required in the calculation since both the elevation and the altitude are measured from sea level.