The efficiency of a pulley is the number of units of work needed to lift the object, divided by the number of units of work being exerted on the pulley. Assuming there is no friction, these two quantities are always equal, giving the pulley an efficiency of 1.
We can verify this using the formula Work = Force * distance. The force needed to lift the load is is 75 N * 40 m = 3000 J. Again assuming no friction, the force exerted on the pulley is 3000 J. Additionally, by solving 120 N * x = 3000 for x, we can determine that the length of rope that the pulling action must travel is 25 m.
(If there is any friction in the system, it reduces the efficacy of the units of work applied to the pulling action by a multiplier between 0 and 1.)