there are 22
6,561
Assuming leading zeros are not permitted, then: If repeats are not allowed there are 30 possible numbers. If repeats are allowed there are 60 possible numbers.
10,000
Possible solutions - using your rules are:- 11,13,17,31,33,37,71,73 &77
There are twelve possible solutions using the rule you stated... 13,14,17,31,34,37,41,43,47,71,73 & 74
There are 60480 numbers.
The largest integer that can be formed by using the digits only once each and simply arranging them in some order is 8740. Otherwise, using exponents, much larger numbers are possible.
There are 4^6 = 4096 such numbers.
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.
8.
125