One January morning, a warm chinook wind rapidly raised the temperature in Spearfish, South Dakota, from below freezing to +11.1° C. As the chinook died away, the temperature fell to -20.8° C in 25.3 minutes. Suppose that a(n) 18-m aluminum flagpole were subjected to this temperature change. Find the average speed at which its height would decrease, assuming the flagpole responded instantaneously to the changing temperature.