> Q.この日記を書いた人は、数学のテストについてどう思いましたか。  
>  A.この日記を書いた人は、数学のテストはとてもむずかしかったと思いました。

This is a test question and a sample of a student response to it. I am
wondering if the form 思いました used with この日記を書いた人 clearly means that the writer
of the diary thinks this (not me)? Should we actually use 思っていました, as in
思っています to refer to "(another) person thinks this" (as opposed to me thinking
it with 思います). So far, I am judging it to be acceptable but I'm not completely
sure.
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question
verbs tense aspect
思っています・思います・思いました
> Q.この日記{にっき}を書{か}いた人{ひと}は、数学{すうがく}のテストについてどう思{おも}いましたか。
>
> A.この日記を書いた人は、数学のテストはとてもむずかしかったと思いました。

In this context, using 「思{おも}いました」 would be considerably more natural than
using 「思っていました」. At least, the former would be the native speakers' choice
much of the time.

Why so? It is because both the question and the answer are about someone's
_**impression**_ of a thing (math test in this case). To tell about one's
impression of a thing, we mostly use 「思う」 for present and 「思った」 for past.
「思いました」 is just a politer form of 「思った」 as you know.

To correctly use 「思っている/います」 or 「思っていた/いました」, you would need to **keep
thinking of something for an extended period of time**. How much time would it
take one to realize that a certain math test is difficult? Most likely, only a
minute or so, agreed?

If you, however, are talking in retrospect about **many or all of the math
tests** you had to take in high school, it would be more natural to use
「思っていた/いました」. That is, of course, because you kept thinking for a few years
that the math tests were difficult.
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answer