Question

If steam enters a turbine at 600°K and is exhausted at 200°K, calculate the efficiency of the engine.

Answer 1

Answer: e = 3.333% or 33 1/3%
                       ^
                  repeated

Identify the given information.
Work input (Wi)= 600 K
Work output (Wo) = 200 K

Choose which formula you should use. In this case, you are finding efficiency.
e =  (Wo/Wi) x 100%

Substitute and solve. 
e = (200 K/600 K) X 100%
e = 0.333 X 100%
e = 33.333% or 33 1/3%

Answer 2

Answer: If in you machine you put some heat X, the machine will return some work W and some heat Y. In this case, X = 600k and Y = 200k.

The efficiency of a machine is the ratio of heat transformed into work, so

E = W/X = 1 - Y/X = 1 - 200/600 = 1 - 1/3 = 0.66.

So the efficiency of the turbine is of the 66%