Power input problem?

Question

A machine lifts a load of 30kg vertically upwards at a constant speed of 1.2 m/s. If the machine is 75% efficient what power input does the machine require?

I've been using the formulas:

F = mg

and P = Fv

then i multiplied that answer by 0.75, and i got it wrong. The actal answer is 470 W. Please show me how to get this. Thanks :)

Answer 1

You need to divide by 0.75. If it is 75% efficient, that means you have to put more energy in than you get back out in the way of useful work done.