Question

how many 3 digit number can be formed by using the digit 1 to 9 if no digit is repeated ?​

Answer 1

There are 3 blanks

_ _ _  

The last blank can be filled with any digit from 1 to 9. so there are 9 ways

The blank before that will be filled in 8 ways because repition is not allowed

and the first blank will be filled in 7 ways

so, This can be done in 9×8×7=504 ways

Answer 2

there are 729 3digit numbers can be formed