You can put this solution on YOUR website! Given that we are looking for 6-digit numbers, the first digit cannot be 0, so there are 5 possibilities for the first digit.
After using 1 digit, there are 5 remaining possibilities for the 2nd digit.
After using 2 digits, there are 4 remaining possibilities for the 3rd digit.
After using 3 digits, there are 3 remaining possibilities for the 4th digit.
After using 4 digits, there are 2 remaining possibilities for the 5th digit.
After using 5 digits, there is only 1 remaining possibility for the 6th digit.
So the total number of different 6-digit numbers that can be formed is:
5*5*4*3*2*1 = 600