There are 16 4-digit numbers that can be made up from 9 and 6.
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what is the least possible sum of two 4-digit numbers?what is the least possible sum of two 4-digit numbers?
The first digit can have 5 possible numbers, the second digit can have 4, the third 3, the fourth 2. 5
Add the two greatest possible four digit numbers. 9999 + 9999
There are 4 possible numbers if the digits are not repeated; 18 if they are. Those are 3-digit numbers, assuming that zero would not be a leading digit. If zero is allowed for a leading digit, then you can have 6 for the non repeated, and 27 if repetition.
There are 210 4 digit combinations and 5040 different 4 digit codes.