The point of that exercise was to show whether you knew how to do it. Things like division and multiplication etc are so basic that you should know how to do it even if you haven't done it for years.

As for mental arithmetic, its main advantage is speed. Faster to mentally solve and continue along your train of thought/flow of conversation than to pull out a calculator. Physical actions will never be faster than speed of thought.

With only a little practice, I found I can add up a column of 4-digit numbers in seconds. Faster than you can whip out your calculator app.

Its the impression that computers are easier, that gets me. Why not practice for 2 minutes and learn a better way? Instead of limping along with the computer all your life.