What beat frequency will the observer detect?

[mortho]

Homework Statement

Two trains emit whistles of the same frequency, 282 Hz. If one train is at rest and the other is traveling at 45 km/h away from an observer at rest, what beat frequency will the observer detect?

Homework Equations

The Attempt at a Solution

I used listener behind formula and got an answer of 544 Hz because i doubled the Frequency source. The answer is wrong, please help me!Thanks!

 

[lavalamp]

Why don't you show your working out.

 

[mortho]

While i was waiting i looked over it again and this is what i did for another answer i got

F(listener behind)=F(of source) [v/v+v(source)]

I did one for each since there are two trains.
For the train on rest, i did F(listner behind)=282*[343/(343+0)] which is 282 Hz

For the train going 12.5 m/s i did, F(lb)=282*[343/(343+12.5)] and got 272 Hz

So then i subtracted 282-272 and got 10 Hz, is this the answer?

 

[lavalamp]

That's the one.

 

[mortho]

Thanks! I'm also having major trouble with an intensity problem, can you help me with that? Thanks!