Explanation:
Power of electric kettle, P = 1 kW
Voltage, V = 220 V
(a) Electric power is given by the formula as follows :

R is resistance

(b) When connected to a 220 V supply, it takes 3 minutes for the water in the kettle to reach boiling point. 
Energy supplied is given by :

P is power, 

 
        
             
        
        
        
Answer:
c. about 1/10 as great.
Explanation:
While jumping form a certain height when we bend our knees upon reaching  the ground such that the time taken to come to complete rest is increased by 10 times then the impact force gets reduced to one-tenth of the initial value when we would not do so.
This is in accordance with the Newton's second law of motion which states that the rate of change in velocity is directly proportional to the force applied on the body.
Mathematically:


since mass is constant

when 
then, 


 the body will experience the tenth part of the maximum force.
where:
 represents the rate of change in dependent quantity with respect to time
 momentum
 mass of the person jumping
 velocity of the body while hitting the ground.
 
        
             
        
        
        
Efficiency = useful energy out / total energy in x 100
= 100/400 x 100
=0.25 x 100
= 25%
25%