The efficiency for converting energy to work in muscles is not 100%. It is typically in a 18% to 26% range and varies with the individiual and the muscle.
See
http://en.wikipedia.org/wiki/Food_energy
They may expect you to assume 100% efficiency to answer your question. If so, "they" (whoever prepared your course material) are very wrong.
During a workout, a person repeatedly lifts a 13 lb barbell through a distance of 1.5 ft. How many "reps" of this lift are required to burn off 100 Cal?
2 answers
Since the body consumes food energy at a "1 MET" level (which also varies with the individual) even when NOT doing work, if you wait long enough, zero reps are necessary to burn 100 Calories