Since you have six digits and are asked for six digit numbers, this is

equivalent to asking how many permutations are there of six distinct

objects.

The number of permutations of six distinct objects is 6!=720.

You can see that this is the case as the first digit in the perm can be

any of the six, the second any of the remaining 5, the third any of the

remaining 4, ... The total number of permutations is the product of

these, that is 6x5x4x3x2x1=6!

RonL