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.