An airplane, flying horizontally at an altitude of 3 miles, passes directly over an observer. If the constant speed of the airplane is 240 miles per hour, how fast is its distance from the observer increasing 45 seconds later? Hint: Note that in 45 seconds (three fourths times StartFraction 1 Over 60 EndFraction equals StartFraction 1 Over 80 EndFraction hour ), the airplane goes 3 miles.