A plane makes a trip from New York to Washington and back to New York. Call the distance between the two cities 200 miles and the speed of the plane 100 miles per hour. Then the time required for the round trip, ignoring stops, is 4 hours. Now suppose there is a strong wind which blows throughout the entire trip with the same speed and in the same direction-from New York directly toward Washington, say. Then the tail wind on the way south will speed up the plane to the same extent that the head wind will retard it on the way north.
In other words, both the average speed of the plane and the time for the round trip will be independent of the speed of the wind. But this means that the plane can still make the trip in 4 hours even though the speed of the wind is greater than that of the plane, in which case the plane would be blown backward on the trip from Washington to New York!