Question

How many 3-digit numbers can be formed by using the digits 1 to 9 if no digit s repeated?

Answer 1

There are 9 ways to pick the 1st digit For each of those 9 ways there are 8 ways to choose the second digit. That's 9 x 8 or 72 ways to pick the first two digits. For each of those 72 ways there are 7 ways to choose the third digit. That's 72 $\times$ 7 ways or 504 ways to pick all three digits.