The mechanical advantage is defined as the ratio between the force produced by a machine and the force applied in input:
[tex]MA= \frac{F_{out}}{F_{in}} [/tex]
For the crowbar of the problem, the force applied in input is 40 N, while the force produced in output is equal to the weight of the rock that is lifted, so 400 N. Therefore, the mechanical advantage is
[tex]MA= \frac{400 N}{40 N}=10 [/tex]
