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.