In my opinion, this is a very badly worded problem without a conceptually
clear picture of what is going on. Knowing the answer, I can back out what
the question really was, but let's use the author's confused statement to
try and learn something ourselves. First of all, the best answer to the
problem as written is zero. That is because the total entropy change
(the total change in entropy of the system and its surroundings) for a reversible
process is always zero. But what is really being asked is, what is the entropy
change for dispersing a given amount of heat into the block of iron. Remember
that we are transfering heat from some unspecified hot source, by some unspecified
reversible mechanism to the block of iron. To say that the process is reversible
means in effect that the maximum amount of work was performed in the process
of the heat transfer. The most common example of such a mechanism is the
Carnot cycle, which you can read about in your book. Clearly, there must
be an equivalent, but opposite entropy change in the heat source. Now the
problem is simply to decide what the entropy change is in the cold reservoire
of a Carnot cycle. The book states that this is just the heat transferred
by a reversible process divided by the temperature of the cold reserviour.
Answer: a) 92 J/K, b) 67 J/K