From someone who has taken physics this is, True
        
             
        
        
        
The ideal concept for solving this question is based on the Doppler effect, for which it is indicated that the source's listening frequency changes as the distance and the relative speed between the receiver and the transmitter are also changed. However, if the relative velocity between the two objects is zero as in the particular case presented (since both travel at 75km / h) we have that there will be no change in frequency.
Therefore the frequency that I hear and that my sister would listen would be the same.
 
        
             
        
        
        
KE = 1/2 * m * v2
We have to rearrange this so the subject is mass. (because the question asks for the mass of the object) :
Mass = (2 * KE) / v
Now input the values into this equation to get your answer :
Mass = (2 * KE) / v
Mass = (2 * 480) / 8
Mass = 960 / 8
Mass = 120
        
                    
             
        
        
        
It's easier for you to solve these than to try and read my solutions if I solve them.
Use this magic formula:
(period) · (frequency)  =  1
If you handle the magic formula carefully and correctly, you can get these facts out of it:
--  Period =  1 / frequency
-- Frequency =  1 / period
Use the first one to solve #1.
Use the second one to solve #2.