An airplane flies 500 mi/hr in still air. Flying with the jet stream, the plane travels 1200 my from City A to City B. The plane then returns to City A,flying against the jet stream. The round-tip flight time is 5 h. What is the speed of the jet steam in miles per hour?

Let be x the speed of jet stream (in mph):

in the going out trip the plane travels 1200 miles at (500+x) mph, thus travel time is
\frac{1200}{500+x}; in the return trip the plane travels 1200 miles at (500-x) mph,
thus travel time is \frac{1200}{500-x}. Round trip time is 5h, thus

\frac{1200}{500+x}+\frac{1200}{500-x}=5

Solve in x.