He's just fooling you

You should ask him how to calculate this kind of question.

Because 1 * 2 * 3 * * * 999 = 999!

! It was factorial, and the result was abnormal.

1! = 1

2! =2

3! =6

4! =24

5! = 120

6! =720

7! =5040

8! =43200

The latter figure is irregular.