The mechanical advantage of a pulley system is calculated as the ratio of output force to input force. In this case, the output force is the weight of the load being lifted, which is 120 N, and the input force is the force applied to the pulley system, which is 20 N. Therefore, the mechanical advantage of the pulley system is:
Mechanical advantage = output force / input force
Mechanical advantage = 120 N / 20 N
Mechanical advantage = 6
Therefore, the mechanical advantage of the pulley system is 6. This means that for every 1 unit of force applied to the pulley system, the load is lifted with 6 units of force.