Kinetic energy = (1/2) (mass) (speed)²
The rock's kinetic energy is not  
                           (1/2) (4 kg) (10 m/s)²
                      =   (1/2) (4 kg) (100 m²/s²)
                      =         200 Joules .
It may be more, or it may be less.  The only thing 
we can be sure of is that it is not 200 Joules.
        
             
        
        
        
If the machine is 100% efficient, then its 
Mechanical Advantage is (450/50) = 9 .
If the machine is less than 100% efficient, 
then the MA is more than 9 .