Calculate the mechanical advantage of a block and tackle that requires a force of 20N to lift a rock that weighs 40N one meter off the ground

The mechanical advantage is

(output force)

divided by

(input force).

If you can lift a 40N rock with a 20N force, then whatever you're

using to do the job has a mechanical advantage of (40/20) = 2.

The distance you lift the rock off the ground is irrelevant.