Plz ! a small tractor is 12% efficient at producing useful output from input fuel. how many joules of input fuel energy will this tractor need to produce 1000 j of useful output?

                           Useful output  =  (efficiency) x (input energy)

                                   (1000 J)  =  (0.12)  x  (input energy)

Divide each side
by  0.12 :                      1,000 J / 0.12  =  input energy

                                     Input energy =  (8,333 and 1/3) J

This is what a calorimeter is used for simpler ex, to measure calories in food.