Quandaries and Queries
Who is asking: Other
I am not sure if you just want to count the number of possible combinations or if you want to list them all. If you want to count them you can think of it this way.
Choose the first digit. You can do this in three ways, the digit can be 1, 2 or 9. Regardless of which digit you place first there are three choices for the second digit, again 1, 2 or 9. Thus for a two digit number you have 3 3 = 9 choices. Now the third digit. Again, regardless which of the 9 two digit numbers you have so far there are three choices for the third digit and hence 3 3 3 = 27 possibilities for a three digit number. Using the same argument there are 3 3 3 3 = 81 possible four digit numbers using the digits 1, 2 and 9.
To list all 81 possibilities do so in an orderly way. I wouls start with 1111 and proceed like this