With what speed must you approach a source of sound to observe a 25% change in frequency?

Sound source is at rest, you are moving with velocity v, f = frequency, c = speed of sound:

f = f0(1 + v/c)

115 = 100(1 + v/343)
115 = 100 + 100v/343
15 = 100v/343
v = 15*343/100
v = 51,45 m/s

Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere. the winds blow from west to east in jet streams but the flow often shifts to the north and south. jet streams follow the boundaries between hot and cold air. the earth's rotation is responsible for the jet stream as well.