Hi,
Going north with jet stream, a plane makes a 1200 mile trip in 3 hours.
Going south against the jet stream, the plane makes the trip in 4 hours.
D = rt or D/t = r
1200/3 = r + s
1200/4 = r - s
400 = r + s
300 = r - s
700 = 2r
r = 350mph, speed of plane and speed of jet stream is 50mph (350+s = 400)