How many 4 digits codes are possible using the numbers 0 -9 only once?

The codes have to be prepared using the numbers 0-9 and are 4
digit long. Also, each of the numbers can be used only once

We have
10 choices for the first digit.

As there are no repetitions, there
are 9 choices for the second digit, 8 choices for the third digit and 7 choices for the fourth
digit.

In all, the number of codes that can be created are 10*9*8*7
= 5040

The number of 4 digit codes possible using the
numbers 0-9 only once is 5040.