Factorial calculate fast

Question

I have an exercise that has factorial and as I know

$10!$

I must do it $10 \cdot 1 + 10 \cdot 2 \ldots$ etc and I don't know how I see this solution. He just written the results,or calculates with another way,faster way? I will use a computer to calculate it but i don't know how he fast did it. \begin{align*} P(n,r) & = \frac{n!}{(n-r)!}\\ & = \frac{10!}{(10 - 3)!}\\ & = 10 \times 9 \times 8\\ & = 720 \end{align*}

Answer 1

Notice that $$10!=10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1$$ while: $$7!=7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1$$ hence it is clear that $$\frac{10!}{7!}=10\cdot9\cdot8=720$$