How do you find mechanical advantage of a pulley?

A single pulley changes the direction of the effort, but it has

no mechanical advantage. The output force is the same as

the input force, so we'd say that the mechanical advantage is 1.

If there are two pulleys, with the rope going up and down and

around between them several times on its way from the effort

to the load, an arrangement that I think is called 'block and tackle',

then the mechanical advantage turns out to be the number of

strands of rope that are supporting the load.