How many 3-digit numbers can be formed by using the digits 1 to 9 if no digit is repeated?
A three digit number is to be formed from given 9 digits 1,2,3,4,5,6,7,8,9.
□□□HTO⇒Now, there are
9 ways to fill
Ones place.
⇒Since repetition is not allowed, so Tens place can be filled by the remaining 8 digits.
So, tens place can be filled in 8 ways.
⇒Similarly, to fill Hundreds of places, we have 7 digits remaining.
So, hundreds can be filled by 7 ways.
⇒So, the required number of ways in which three-digit numbers can be formed from the given digits is 9×8×7=504