If you have 9 numbers how many different 6 digit combinations?

We treat this as a permutations and combinations problem... as there are two possible 'rules' involved.

If we are allowed to repeat a digit (such as 456537 - which as two 5s in it) then there are 9 options for every digit. So 9 x 9 x 9 x 9 x 9 x 9 = 531441 different six-digit numbers from a choice of 9.

If we cannot repeat a digit, as we did above, thee need to pick the first number - and it can be any of the 9.. then the second number can be any of 8, and the third can be any of 7 and so on.... this is called permutations and we end up with a multiplication like before but with each figure lower than the one before it.... 9 * 8 * 7 * 6 * 5 * 4 = 60480 different permutations of six digits from a choice of 9.