Answer:
86.6 km/h
Step-by-step explanation:
= Velocity of Plane = 600 km/h
a = Distance Plane travels
= Vertical velocity of Plane = 0
b = Altitude of plane = 10 km
c = Distance between town and plane = 20 km

From Pythogoras theorem
a²+b² = c²
Now, differentiating with respect to time

The rate at which the distance from the plane to Quinton is increasing 86.6 km/h