It is possible to create a 3-digit number, without repeated digits so the probability is 1.
6561
The largest ten digit number with no repeated digits is '9876543210'.
98765432
The largest number with no repeated digits is 9,876,543,210 although it is 10 digits long, the largest 3 digit number with no repeated digits is 987
987654
1023456789
The largest ten digit number with no repeated digits is '9876543210'.
I think its more then 20
None of the digits can be 10, so the probability is 0.
The greatest 4-digit number with no repeated digits is... 9876
It is 1. Think of any number between 1 and 10 million.