Answers not hidden
Right now the answers and solutions are in plain sight right under the questions. Ideally they'd be hidden somewhere so that the reader can think for a bit before reading it. I'm not sure what the best way to do this is, perhaps having a separate directory for them? It may make sense to do that after the material is a bit more finalized though.
Could you use collapsable headers for this?
Based on this:
Any markdown heading cell (that is, one which begins with 1-6 # characters), becomes collapsible once rendered.
It seems like that extension is something that only allows you to collapse heading when rendering the notebook locally, so it wouldn't affect the way it's rendered on GitHub.