If you have to apply 40 N of force on a crowbar to lift a rock that weighs 400 N what is the actual mechanical advantage of the crowbar?

The mechanical advantage of the crowbar is 10. This is calculated by dividing the load force (400 N) by the effort force (40 N). In this case, the mechanical advantage shows that the crowbar amplifies your force by a factor of 10 to lift the rock.