How Many 4-digit Numbers Can Be Formed Using The Digits 0,1,2,3,4,5,6 If Repetition Of Digits Is Not Allowed?

There are (7!)/(7-4)! = 7*6*5*4 permutations of 7 numbers taken 4 at a time. That's 840. Some of the numbers will have a leading zero.

If you don't allow a leading zero, there are 6*6*5*4 = 720 possible 4-digit numbers.

The first couple are 0123, 0124, ...
The last couple are ..., 6542, 6543.

The set of numbers not having leading zeros begins with 1023, 1024, ...

12345678

If numbers can be repeated how many 4 digit numbers can be formed using the numbers 4,5,7,8,9