There’s 6 numbers that can be used.
The first digit can be any of the 6
The 2nd digit can be any of the 5 left
The 3rd digit can be any of the 4 left
The 4th did fit can be any of the 3 left
The 5th digit can be any of the 3 left
The 6th digit would be the last number left.
Total combinations = 6 x 5 x 4 x 3 x 2 x 1 = 720
