This is permutations with repetition. The answer is 4^4 = 256 total permutations. Since 2 of the digits used are odd (and 2 are even), then half of the possibilities will be odd: 128 odd numbers.
500
It is 120 if the digits cannot be repeated.
4*3*2*1 = 24
That would be 4! = 4 * 3 * 2 * 1 = 24
6*5*4*3*2*1=720
500
24
1 set
It is 120 if the digits cannot be repeated.
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
This permutation problem depends on whether the numbers are allowed to be repeated. If they are, there are a possible 9999 numbers, starting with 0001 and running sequentially to 9999. If they are not allowed to be repeated, there are a possible 5040 combinations.
3x3x3=27
6*6*3 = 108 numbers.
36
4*3*2*1 = 24
6*5*4*3*2*1=720 possible numbers
48 of them if digits may not be repeated. 100 if they can.