As we know that mechanical advantage is defined as the ratio of output force and input force for the pulley system
Here we know that it required 10 lb of input force in order to lift the block of 500 lb so here output force is 500 lb
now we have
[tex]MA = \frac{F_{out}}{F_{in}}[/tex]
here we have
[tex]F_{out} = 500 lb[/tex]
[tex]F_{in} = 10 lb[/tex]
now from above formula we have
[tex]MA = \frac{500}{10}[/tex]
[tex]MA = 50[/tex]
so mechanical advantage of this system is 50