How many 5 digit numbers are there with distinct numbers?

Answer 1

I suspect you mean "without repeated digits", and I'll answer it that way.

Here's how I would construct all the 5-digit numbers without repeated digits:

The first digit can be any one of 9 (1 thru 9 but not zero). For each of these . . .

The second digit can be any one of 9 (zero thru 9 but not the same as the first one). For each of these . . .

The third digit can be any one of the remaining 8. For each of these . . .

The fourth digit can be any one of the remaining 7. For each of these . . .

The fifth digit can be any one of the remaining 6.

Total number of possibilities = (9 x 9 x 8 x 7 x 6) = 27,216