I am joining a maths competition and recently I am preparing for it. I came across a question that asks me to fill the blank of a number:
And this number is the product of $15!= 15\times 14\times 13\times 12...\times 1$.
The competition doesn't allow to use a calculator, so I am wondering how to multiply these without calculator. Can I have a solution that can solve a similar question and also the solution to this question? Tqvm