How do I use the Doppler effect equation when the direction of motion is unknown? This was one of the problems on one of the AP Physics Practice tests I was working on, when I came across a small discrepancy:

A bat named Reggie hunts insects in a cave. Bats use sound waves to locate their prey. Reggie is attempting to determine the location and speed of a tasty moth by emitting $30\ \mathrm{kHz}$ sound waves and listening for the reflection. The speed of sound in the air of the cave is $350\ \mathrm{m/s}$. Assume all motion is one-dimensional.
The moth, flying north at $25\ \mathrm{m/s}$, is struck by waves with a frequency of $30.2\ \mathrm{kHz}$. What is Reggie's velocity?

Since the Doppler Effect is $f_1 = f_0(\frac{v-v_o}{v-v_s})$ when the objects are moving in the positive direction, but $f_1 = f_0(\frac{v+v_o}{v+v_s})$ if they are both moving in the negative direction. Since it is not specified, how would you solve for the velocity of Reggie (the bat)?
 A: The received frequency $f_r$ and the source frequency $f_s$ are related as follows 
$$f_r = f_s\left(\dfrac{v+v_r}{v+v_s}\right)$$   
where $c$ is the speed of waves in the medium, $v_r$ is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source (and negative in the other direction and $v_s$ source relative to the medium; positive if the source is moving away from the receiver (and negative in the other direction).  
Reggie is the source and the moth is the receiver.
There are four possibilities:
1 Reggie and the moth are moving towards one another,
2 Reggie is trying to move away from the moth which is trying to moving towards him,
3 Reggie is trying to move towards the moth which is trying to moving away from him, and
4 Reggie and the moth are moving away from one another.  
For possibilities 1 and 2 it does not matter which direction you choose for the Reggie because if you chose the wrong direction you will get a negative value for $v_s$.  
For possibilities 3 and 4 you will have to do the algebra and see if the answer you get is reasonable although you know it cannot be possibility 4 because the moth receives a higher frequency than Reggie emits.
A: Since the bat and the moth moves uni dimensionally and since echolocation takes place through the mouth of the bat we can say that the direction of sound and the direction of motion of the bat are the same. So the direction  should be positivef1=f0(v−vo/v−vs)
