Question

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

Answer 1

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