I recently wrapped up an interview with a very young and unprepared internship candidate. He got a lot of the questions wrong, even the very easy ones, and would frequently inquire about how to solve the problems.
While I quickly realised that he was not an acceptable candidate, even in the slightest, I decided to run through the entire set of questions, as usual, that I'd prepared for someone who claimed to have the experience that he put on his résumé, which included several previous internships.
We finished just a bit over our allotted time, yet I had spent most of the interview giving him tips on how to solve the problems, and then (usually) having to explain the correct answer. I felt as though this was the right thing to do while I was conducting the interview, however, looking back, I wonder if it was worth spending so much time on a candidate who had no chance of even being considered, and didn't seem to really understand my explanations anyways.
I'm new to interviewing possible interns, and I certainly wasn't ready to screen such unprepared candidates, so I'm wondering if there exists a best way handle these situations as they arise, without being rude, or cutting the interview too short.