I think you're being unfair to yourself if you're calling this cheating. Also FWIW, I interviewed with Microsoft at my college, got asked the very same questio

I think you're being unfair to yourself if you're calling this cheating. Also FWIW, I interviewed with Microsoft at my college, got asked the very same question as the OP and actually solved it the same way. I knew the formula for the sum and the rest just came to me at that moment. I was flown to Redmond, given series of interviews and an offer in OSG. Even though it was my dream to work on Windows kernel growing up, in the last 4-5 years, I had become a FOSS Linux guy so I turned down their offer, much to the surprise of the recruiter whom I had shared this was a life-long dream, and took an entirely different direction with my career and life. Worked out nicely. I am sure I'd be very happy in Microsoft but maybe I am happier.