Question

A heat engine does 200 J of work per cycle while exhausting 400 J of waste heat. what is the engine's thermal efficiency?

Answer 1

The thermal efficiency of an engine is
N=W/Q
where
W is the work done by the engine
Q is the heat absorbed by the engine to do the work

In this problem, the work done by the engine is W=200 J, while the heat exhausted is Q=400 J, so the efficiency of the machine is
N=200J/400J=0.50=50%
I hope this helps!