Yes, in this context the two are interchangeable. From the dictionary:
- difficulty: a difficult situation, matter, or person
- puzzle to be solved: a question or puzzle that needs to be solved
- statement requiring mathematical solution: a statement or proposition requiring an algebraic, geometric, or other mathematical solution
- subject of concern: something for discussion or of general concern
- main subject: the central or most important topic in a discussion or debate
- legal matter in dispute: a legal matter in a dispute between two parties
The bolded portions are most applicable to the context you gave.
To compare, here are two contexts where they wouldn't be interchangeable. There are others but for an exhaustive list compare the two entries in any handy dictionary.
This magazine issue is two years old!
Please answer the first problem found on page 42.