Question: how many combinations with 6 numbers without repeating?
Answer: Similarly, if all the digits from 1 to 9 were allowed (with no digit repeated), then the answer would be 9*8*7*6*5*4 = 60480. The tricky part comes when the digit 0 is allowed: suppose that you wanted to form a 6-digit number using digits from 0,1,...,9 without repeating a digit.