Amachine requires 2,000 joules to raise a 20. kilogram block a distance of 6.0 meters. how efficient is the machine?

Potential energy = (mass) x (gravity) x (height) .

To raise the 20kg block 6 meters, you need to give it

                         (20 kg) x (9.8 m/s²) x (6 m) = 1,176 joules

of potential energy.

Your machine takes 2,000 joules of energy to give the block
the 1,176 joules that it takes.

Your machine is only  (1176 / 2000) = 58.8% efficient .

The other 'missing' 824 joules of energy doesn't just disappear.
It heats the machine, and the air around it.  The final result is
that when you use that machine to lift blocks, you have to use
even MORE energy, to blow cool air on the machine and prevent
it from overheating and breaking.