Using your rule - essentially, it's the simplified sum of 9999999-2000000 which is a total of 7999999 numbers !
81
It is 120 if the digits cannot be repeated.
There are 8*9*8*7*6*5*4 = 483840 numbers.
823543
7
81
1 set
It is 120 if the digits cannot be repeated.
There are 8*9*8*7*6*5*4 = 483840 numbers.
823543
7
There are 5! = 120 such numbers.
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
27 three digit numbers from the digits 3, 5, 7 including repetitions.
27 if the digits can be repeated. 6 if not.
3x3x3=27
24