A power specification formula is to be derived for electric motors which drive c

Question

A power specification formula is to be derived for electric motors which drive conveyor belts moving solid material at different rates to different heights and distances. Denoting the efficiency of the motors by ? and neglecting the power needed to drive the belt itself, derive a formula (a) in the SI system of units, for the power P in kW, in terms of the mass flow rate m in kg/h, the height b, and the horizontal distance l in meters, and (b) in U.S. customary units, for the power in hp, in terms of the mass flow rate m in tons/h, and the height b and horizontal distance l in feet.

Cramster says the answers are:
a.) 0.278 x 10^(-6) mgb/?
b.) 1.010 x 10^(-3) Wb/?

My approach has been starting with P=(mg)b/t but how does efficiency come into this? Is there another way?

Explanation / Answer

(A) Power = [(mg)b]/3600(?) <---Nm/s

Power = [(mgb)/(3600)(1000)(?)] <----kW

Power = (2.78 * 10^-3) [(mgb)/?] <----kW

(B) Power = [[(W tons/h)(2000 lb/ton)(b ft)]/[(3600 s/h)(?)(550 ft/lbs)]] <----- hp

Power = (1.01 * 10^-3)[(Wb)/?] <---- hp

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