If the digits are allowed to be repeated, there are six choices for each of the four digits
in the result, so the answer is 6 x 6 x 6 x 6 = 1296.
If the digits may not be repeated, the number of choices decreases with each successive digit, so there are 6 x 5 x 4 x 3 = 360 possibilities.
The word "combination" is, strictly speaking, a choice without repetition in which the order of selection does not matter (e.g., 1234 is considered the same as 4321). Since each of the 360 permutations counted above contains every combination in 24 possible orders, there are just 360/24 = 15 possible combinations.
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You can make 5 combinations of 1 number, 10 combinations of 2 numbers, 10 combinations of 3 numbers, 5 combinations of 4 numbers, and 1 combinations of 5 number. 31 in all.
There are 24C12 = 24!/[12!*12!] = 2,704,156 combinations.
120 5x4x3x2x1=120
The answer will depend on how many digits there are in each of the 30 numbers. If the 30 numbers are all 6-digit numbers then the answer is NONE! If the 30 numbers are the first 30 counting numbers then there are 126 combinations of five 1-digit numbers, 1764 combinations of three 1-digit numbers and one 2-digit number, and 1710 combinations of one 1-digit number and two 2-digit numbers. That makes a total of 3600 5-digit combinations.
It depends. If you can only use each number once, there are 720 combinations. If you can use numbers multiple times, then there are 1000 combinations, by using all numbers from 000 to 999.