all 6 digit combinations using only 1,3,7,9

Question

can use any of the numbers multiple times but must use all 4 digits at least once

Answer 1

There are four different numbers and any can be repeated so using these digits to produce a 6-digit number means that there is a choice of 4 digits for every place. This means that we have 4^6=4,096 different numbers. It's similar to asking how many 6-digit decimal numbers can we produce using 0 to 9, when the answer is 10^6, everything from zero to 999,999. But please don't ask me to list them all! To make a start: 111111, 111113, 111117, 1111119, 111131, 111133, 111137, ... See what I mean? Eventually there will numbers containing all four digits.

Answer 2

All 6 digit combinations using only 1,3,7,9

Total numbers =4
4^6=4096

There are 4096 numbers