'question on' versus 'question about'

I see none.

There does seem to be the subtlest difference - which is why there are two ways of saying this. For example, if a student raises his hand and says, "I have a question on problem 5 in the homework assignment for yesterday.", this sounds very standard. He simply has a question on one of the problems.

However, if he says instead, "I have a question about prob. 5 in the homewk. assignment f

I still see none.

Hi Mister Micawber,

Thank you so much for your reply.

Best,

TS

Hi Anonymous,

Thank you very much for your reply too. But if I'm not mistaken, the result is the same, there's a question to be answered, isn't there?

Best,

TS

http://www.learnersdictionary.com/search/toughie
The last question on the test was a toughie.

Would it be correct to say "about" in place of

No, because 'test' is not the topic of the question, but its location. These are OK and equivalent:

The last question about the American Constitution was a toughie.

The last question on the American Constitution was a toughie.

Thank you, Mister Micawber.