50 points to who answers this question in a clear and simple explanation : to what hight will a mass of weight of 500 n be raised in 20 seconds by a motor using 4kw of power?

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".)  

Height = 160 meters

it was given as a lesson and guide for prayer.

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form