An Interesting Interview Question.

Very recently, one of my students had an interview for an internship at a very prestigious investment bank. They were asked the following question as part of the interview. Can you solve it?

The question was reportedly given in the following way:

Make sure to give the problem a go yourself before reading on.

The student, let’s call her Amanda, admitted they were feeling a bit down as they were sure they had gotten the wrong answer. I told her not to worry too much about this as generally these questions are not asked to see if an applicant gets the right answer, but instead to see how they approach the question. I left Amanda, a little more upbeat than I found her, promising to think about the problem later and devise a solution.

Admittedly, at first, I didn’t see the purpose of asking about this particular problem. It was only when developing a solution that I think I cracked the reasoning behind it. To make sure I was right, the next day I asked Amanda if the interviewer had explicitly banned the use of a calculator, she, surprised at my question, confirmed they had indeed. This left me pretty confident my logic was correct, let’s see if you agree.

Here’s my thinking on it. From teaching as much as I have, I know exactly how most students would approach this question, and it’s…