Question

How many 3-digit numerical passwords are there if a digit cannot be repeated?

Answer 1

The first digit of the password can be any of the ten digits - 10 possibilities.
A digit cannot be repeated, so the second digit can be any digit except the one you chose for the first digit - 9 possibilities.
The third digit can be any digit except the digits you chose for the first and second digit - 8 possibilities.

Using the rule of product:
$10 \times 9 \times 8=720$

There are 720 passwords.