An airplane flies from Boston to San Francisco (a distance of 5000 km) in the morning, then immediately returns to Boston. The speed of the plane relative to the air is 240 m/s. The wind is blowing at 41 m/s from west to east, so it is "in the face" of the plane on the way to San Francisco and it is a tail wind on the way back.

Question

An airplane flies from Boston to San Francisco (a distance of 5000 km) in the morning, then immediately returns to Boston. The speed of the plane relative to the air is 240 m/s. The wind is blowing at 41 m/s from west to east, so it is "in the face" of the plane on the way to San Francisco and it is a tail wind on the way back. 
(a) What is the average speed of the plane relative to the ground on the way to San Francisco?

(b) What is the average speed relative to the ground on the way back to Boston?

(c) What is the average speed for the entire trip?

Jennifer · Answer

From Boston to San Francisco, the speed of the plane relative to the ground is (240 - 41) = 199 m/s

From San Francisco back to Boston, the speed is (240 + 41) = 281 m/s

c) The average speed is just

(199 + 281)/2