It’s 4. Definitely 4. 99% sure it’s 4.
        
             
        
        
        
Zero is induced in the secondary, and the total output of the transformer consists of smoke.
Transformers only work with AC.
        
             
        
        
        
To develop this problem we will apply the concepts related to the Doppler effect. The frequency of sound perceive by observer changes from source emitting the sound. The frequency received by observer  is more than the frequency emitted by the source. The expression to find the frequency received by the person is,
 is more than the frequency emitted by the source. The expression to find the frequency received by the person is,

 = Frequency of the source
= Frequency of the source
 = Speed of sound
= Speed of sound
 = Speed of source
= Speed of source
The velocity of the ambulance is


Replacing at the expression to frequency of observer we have,


Therefore the frequency receive by observer is 878Hz
 
        
             
        
        
        
For rotational equilibrium of the door we can say that torque due to weight of the door must be counter balanced by the torque of external force

here weight will act at mid point of door so its distance is half of the total distance where force is applied
here we know that

now we will have


so our applied force is 72.5 N
 
        
             
        
        
        
F=K*X,
F=M*a 
M*a=K*X
2.5*9.81=K*0.0276
24.525=K*0.0276
24.525/0.0276=K
K= 888.6 N/m ---- force constant 
assuming 2.5 refers to the new extension, just divide F/ 0.025
to get
981N/m