Find the current to run an electrical motor that has an output of 2.1 horsepower and is 71% efficient. operates at 171 volts answer to nearest tenth of an amp.

Wow !

1 horsepower = 746 watts
2.1 horsepower = (2.1 x 746) = 1,566.6 watts

Since the motor is 71% efficient, 1,566.6 watts = 71% of the input power.

                                             1,566.6 = 0.71 P

Divide each side by 0.71 :     input Power = (1,566.6 / 0.71)  watts
(Let's leave it like that for right now.  The final answer will be more accurate
if we can avoid rounding until the very end.)

                               Input power =(voltage) x (current)

                  (1,566.6/0.71 watts) = (171 volts) x (Current)

Divide each side by  171 volts:    Current = (1,566.6) / (0.71 x 171) = 12.903 Amps.

                                                                                             Current = 12.9 Amperes. 

Check:

               (volts x Amps) x (efficiency) = output power

               (171 x 12.9)(watts) x (71%) x (1 HP / 746 watts) =

               (171 x 12.9 x 0.71) / (746)      HP  =  2.0994 HP

                                               Close enough to 2.1 for Engineers !

                                                                       yay !