How to Calculate Doppler Effect Frequencies for Moving Objects?

[Wen]

I've got 2 qns
1)
Student holding a tuning fork vibrating at 440Hz walk away from a wall with a speed of 1.2m/s. What's the freq of the echo the student can hear from the wall? Speed of sound is 330m/s

Can i just use Freq= [(330-1.2)/(330+1.2)] (440Hz) to solve
2) A car traveling at 10m/s sound its horns, which has a freq of 500Hz and this is heard by another car traveling behind the 1st car in the same direction but at a speed of 20 m/s.The sound can also be heard by the 2nd car by the reflection from the bridge ahead. What frequency will the 2nd car hear? Take speed of sound be 340m/s.

Is the frequency they are asking beat freq?
How do i cal. the freq due to the doppler effect? Must i make use of relative velocity?

 

[Chi Meson]

1) No they don't want the beat frequency, but in this case the walking person is both the source AND the observer (and both source and observer are moving)

2) Same thing; think of the situation as though the observer is the reflection in a mirror attached to the bridge. Use observer direction as it appears in the reflection.