This pulley system provides a 4:1 mechanical advantage. The user is required to apply a force of 25kg to raise this 100kg load, for every 4 metres of rope that the user pulls through the pulley system the load will only be raised by 1 metre.
If we count the amount of ropes that are actually supporting or applying effort on the load, and then divide the loads weight equally between them we should easily be able to calculate the overall mechanical advantage of the pulley system.
In this scenario there are four sections of rope supporting and applying effort to the load. So the loads weight (100kg), divided by the amount of supporting sections of rope (4), should result in each rope supporting one quarter (25kg) of the loads total weight. The rope is then returned to the user via a redirect (pulley D) who applies 25kg of effort to raise the 100kg load.