Find the number of possible choices for a 4-digit PIN if the digits cannot be repeated.

Hello!

The first digit may be any, there are 10 options, 0 to 9.

The second digit may be any except the first digit, so 9 options for it.

The third digit may be any except the first and the second, 8 options.

And for the fourth digit there are 7 options the same way.

Totally there are 10*9*8*7=5040 options for 4-digit PIN.