A train on one track moves in the same direction as a second train on the adjacent track. The first train, which is ahead of the second train and moves with a speed of 28 m/s, blows a horn whose frequency is 125 Hz. If the frequency heard on the second train is 132 Hz, what is its speed? so I tried using the equation: f' = [(1+vo/v)/(1+vs/v)]f (both pluses taking into account the direction of both the observer and the source) where: f'=frequency heard=132 Hz f=actual frequency=125 Hz vo=velocity of observer=unknown vs=velocity of source=28 m/s v=343 m/s so I plugged in the numbers and got 48.78 m/s. The website won't take this as the answer and I'm completely clueless as to what I did wrong... any help?