An airplane flies in a straight line from airport A to airport B and back. It has constant engine speed and no wind.
Will its total flight time increase, decrease or stay the same if throughout both flights there is a constant wind blowing from A to B?
Although at first it may be tempting to assume that the boosting/retardation of the wind cancels out and the flight time remains the same, this is not so.
The plane will spend a greater amount of time flying into headwind because of its slower speed. Thus the overall effect of the wind is that of retardation. Essentially, the retardation of the wind acts for a longer period of time. A direct round-trip will always be faster with no wind rather than a constant wind.