How can one find the mechanical advantage of a pulley?

To calculate the mechanical advantage of a pulley you simply have to count the number of rope sections that support whatever object you are lifting (not counting the rope that is attached to the effort)

Example:

For example, in a one pulley system the MA is 1. In a two pulley system the MA is 2. The more compound the pulley is the easier the load is to lift. If you were lifting a 600kg object with 2 pulleys you would have to use enough effort to pull 300kg to lift the 600kg object! Therefore your mechanical advantage would be 2 (600/300).

               MA= load/effort OR number of rope sections

see earlier