## Precalculus (6th Edition) Blitzer

The speed of the plane over the intercom means, the speed of the plane is measured after the whole effect on the plane with respect to the air. That means the plane speed is measured after the wind effect. Example: Suppose a car is driving at a speed of $60$ km/hr. The speed shown on the speedometer of the car is $50$ km/hr. This means the speed of the wind is $10$ km/hr. which is in the opposite direction of the car. Hence, the speed of the plane is the speed after the effect of the wind.