How many ways to form a 6-digit number which the digits cannot be repeated and the leftmost digit cannot be 0?
I found there're two possible answer:
since 0 is not available to the leftmost digit, so there're only 9 choices and 9 choices on the second digit(since 0 is available) in the second digit, 8 choices on third digit, and so on. The total number of way is: 9*9*8*7*6*5 = 136080
but if I start at the rightmost digit, the answer will be different!!!
There are 10 choices at the rightmost digit, 9 choice at the second-rightmost digit, and so on. There are 6 choices at second-leftmost digit, 4 choices left at the leftmost digit (since 0 is unavailable here). The total number of ways is: 10*9*8*7*6*4 = 120960.
Could anyone explain this?