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

Explanation

Three digit numbers by using digits 1 to 9 with no repetition are to be formed
`rArr` Three places are to be filled with digits 1 to 9 which can be done in `.^(9)P_(3)` ways
`=(9!)/(6!)=(9.8.7.6!)/(6!)=(9) (8) (7) = 504` ways