You have 2 options for the first digit, 2 options for the second digit, etc. ... In total, that gives you 210 combinations.
99999
There are 3,265,920 numbers that can be formed using all the digits. The answer to the question will depend on which one of these.
There are 60480 numbers.
There are 4^6 = 4096 such numbers.
8.
If the digits are all different then 18. Otherwise, 192.
15 of them.
27
Possible solutions - using your rules are:- 11,13,17,31,33,37,71,73 &77
6
There are 2000 such numbers.
9 odd numbers less than 100 can be formed. They are: 3,5,7,35,37,53,57,73 and 75.