You only need to use the right two simple formulas:
Work = (force) x (distance)
-- A mass that weighs 500N is being pulled down by gravity with a force of 500N. (That's what "weighs" means.) If you want to lift it straight up against gravity, you have to lift with a force of 500N.
The WORK you do on the mass is (500N) x (height you raise it to).
Power = (work) / (time)
-- 4,000 watts = (500N x Height) / 20 seconds
From here, the rest is just algebra ... pulling the height out of this equation:
Multiply each side by (20 sec):
(500N x height) = (4,000 watts x 20 sec)
Divide each side by (500N):
Height = (4,000 watts x 20 sec) / (500N)
Height = (4,000 x 20 / 500) (watt x second / Newton)
(Remember that "watt-second" = "Joule",
and "Joule" = "Newton-meter".)
<em>Height = 160 meters</em>
