<span>2.56 sec (t=1/0.39)
f=3.9/10</span>
        
             
        
        
        
Explanation:
 its easy everone can do it .....let me know
 
        
             
        
        
        
Answer:
Speed of the wave is 7.87 m/s. 
Explanation:
It is given that, tapping the surface of a pan of water generates 17.5 waves per second.
We know that the number of waves per second is called the frequency of a wave. 
So, f = 17.5 Hz
Wavelength of each wave,  
 
Speed of the wave is given by :
 
 
 
 
v = 7.87 m/s
So, the speed of the wave is 7.87 m/s. Hence, this is the required solution. 
 
        
             
        
        
        
The force exerted by the magnetic in terms of the magnetic field is,

Where B is the magnetic fied strength and F is the force.
Thus, if the magnetic A has twice magnetic field strength than the magnet B, 
Then,

Thus, the force exerted by the magnet B is,

Thus, the force exerted by the magnet B on magnet A is 50 N. 
The force exerted by the magnet A exerts on the magnet B is exactly 100 N as given.
Hence, the option B is the correct answer.
 
        
             
        
        
        
They do the method 3 times to be sure. Because if you do it once, that could mean anything. If you do it twice, it may or may not have the same result. If you do it 3 times and it matches one of the previous answers, then it's likely that it's correct.