Philosophy.SE lists these questions as on-topic:
We welcome questions involving subjects like:
- epistemology — the nature of knowledge and learning
- ethics — the nature of the right thing to do
- metaphysics — the nature of being and reality
- logic — the nature of reasoning and inference
- aesthetics — the nature of feelings and artistic sentiment
- history of philosophy — the domains and personalities in the past associated with philosophy
- applied philosophy — the more specific disciplines where philosophical methods are applied, such as logic and argumentation, the philosophy of language, political and social philosophy, psychoanalysis, theology, economics, the philosophy of science and mathematics, etc.
There are 2,401 questions tagged "logic", and 327 questions tagged "argumentation"; along with other related tags such as: modal-logic on Philosophy.SE.
Math.SE does have 252 results for the search term "word problems". Math.SE has 33,836 and Mathoverflow.net has 5,664 questions dealing with gr.group-theory (abstract algebra that deals with groups).
Where to ask depends upon whether your question is hypothetical, abstract or absolute.
"River crossing problems" in particular are solved using graph theory which has 16,240 questions, dynamic programming which has 362 questions, integer programming which has 687 questions, or even combinatorics with 38,693 questions on Math.SE.