Say the code only needed to be one number long. Going from 0-9, there are ten digits. Therefore, there are ten unique codes.
Now say the code has to be two numbers long. There are ten possibilities
for the first digit, 0-9. But the second digit only has nine
possibilities, since each number has to be different. (The first number
might be 0, so the second can only go from 1-9) For each of the first
ten possibilities, there are nine possibilities for the second. We
multiply those two numbers together, 10 9, and we get 90.
Now we just extend the logic. The third number only has eight
possibilities, while the fourth number only has seven. 10 9 8 7 =
5040 possible codes. I hope this helped! has a great day! :)
