Human Calculators?

Ok, so the other day in maths, we were doing permutations. You know, like "How many different ways can you arrange the letters in the word 'MEDIAN'?" and stuff like that.
With these questions, the Factorial is a commonly used idea, Which looks like this:
8!
And means this:
8x7x6x5x4x3x2x1

Now, Denis, someone in my class, was sitting beside me (Slightly tautological) And he was doing a question that included factorials (Extremely tautological).