In the question, we have to find the possible number of 6 digit numbers formed by the numbers 4, 5, 6, 7, 8, 9 when repetition of digits is not allowed.

We will use the concept of multiplication because there are six sub jobs dependent on each other because a number appearing on any one place will not appear in any other place.

The number of ways in which we can form six digit numbers with the help of given numbers is 6 × 5 × 4 × 3 × 2 × 1 = 6! = 720

The numbers occurring on first place from the left have 6 choices and when one number is placed then number occurring on the second place from the left will have 5 choices and so on one fewer choice will be available to every next place till one occurs.