I think 问题 doesn't necessarily mean literally a question. 'troubles' or 'problems' should be accepted as a correct answer.
一个 makes this singular in English, so problem (no 's') should be accepted. And "a simple trouble" is just not a phase that's used in English.
简 (jiǎn) = simple/uncomplicated
单 (dān) = bill/list/form/single/only
简单 (jiǎndān) = simple
We saw 单 (dān) before in:
Easy and simple should both be accepted
I am still missing what makes it "not simple" instead of "simple"
簡単 also means easy, I think.
That is just the traditional character of 简.
In Japanese, yes.
Is it necessary to use 一个 in this sentence?
And is it necessary also to use 的 ?