They attract and stick together
To solve this problem we will apply the concepts related to the Doppler effect. According to this concept, it is understood as the increase or decrease of the frequency of a sound wave when the source that produces it and the person who captures it move away from each other or approach each other. Mathematically this can be described as

Here,
= Original frequency
= Velocity of the observer
= Velocity of the speed
Our values are,



Using the previous equation,

Rearrange to find the velocity of the observer

Replacing we have that


Therefore the velocity of the observer is 16.2m/s
The zeroth law of thermodynamics states that if two thermodynamic systems each are in thermal equilibrium with a third, then they are in thermal equilibrium with each other.
As long as the graph is a straight line they both will give same answer. Edit: In the case of errors,I think the most fitting slope line is more accurate method,because taking average is same as drawing a line from initial to final point of the line and taking its slope.
Hope it helps