Question

how many 4 digit number can be formed using the digits 0 to 7 if no digit is to occur more than once in any number

Answer 1

here is your answer..
two digits can be formed..
1. 1256
2. 3407
or u can put it in any order..

Answer 2

There are 9 ways to pick the first digit (it can't be 0)

9 ways to pick the 2nd digit (it can be 0, just not what we picked for the 1st digit)

8 ways to pick the third digit

and 7 ways to pick the fourth digit.

That's 9×9×8×7, or 4536 ways