An airplane flies from Boston to San Francisco (a distance of 5000 km) in the morning and then immediately returns to Boston. The airplane’s speed relative to the air is 250 m/s. The wind is blowing at 50 m/s from west to east, so it is “in the face” of the airplane on the way to San Francisco and it is a tailwind on the way back. 

(a) What is the average speed of the airplane 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? 

(d) Why is the average of the average speeds for the two legs of the trip not equal to the average speed for the entire trip?