Find how many numbers bigger than 2,999 and less than 8,999 can be formed from the odd digits, if no digits can be repeated

Answer 1

the first digit must be 3,5,7
the other digits can be 1,3,5,7,9

since the 1st digit cannot be repeated, that means the last 3 digits must be formed using only 4 of the available choices, giving
4*3*2 choices.

So, since the 1st digit can be chosen 3 ways, there are

3*4*3*2 ways to pick the digits.

Answer 2

72