A classical mechanics problem by Jàfąř ÀĀł

The distance from Boston to San Francisco is 4344 km. However, a flight from Boston to San Francisco is about 6 hours and 20 minutes of flying time (the flying time is the total time minus the time required to take off​ and land), whereas a flight from San Francisco to Boston is only 5 hours and 20 minutes of flying time. The difference is due to the jet stream, which blows across the United States roughly from west to east. If the speed of the airliner relative to the air is a constant, and the jet stream is considered as blowing at a constant speed directly against the airplane as it flies from Boston to San Francisco and directly with the airplane on the return flight, then what is the speed of the jet stream in km/hr?