average time to access a word in memory
This is not the answer, but you may consider it as hints to find it
yourself.

Why do you write 4.95=5 without any explanation. Is that your idea of
perfect solution? Apparently your first solution never accesses the
disk. Your second solution is essentially the same as the first,
presented more abstractedly (hence a bit better in my opinion, but the reasonning may not be explicited enough for what seems to be your level of proficiency), but it
ignores the disk access as much as the first soution.

That may not always be true, but you should always worry at least a
little bit where you are ignoring part of the data of your problem.
You should convince yourself that the data you ignore is


either redundant with the data you use. This means that it is
actually a consequence of the data you are using, and can thus be
taken into acount implicitly through the data you are using;
or irrelevant to the problem. In this case you should preferably
have some argument as to why it is irrelevant. This argument may not
be required in your answer (though it may not hurt to give it), but
you probably at least sketch it for yourself to convince yourself
that you are right in ignoring the corresponding data.
Irrelevance can also include the fact that the corresponding data is negligible in the context of your problem. But that usually demands some argument to convince the reader that it is indeed the case.


Does the data you ignore correspond to such a case?