To calculate the number of 3-digit combinations that can be made from the numbers 1-9, we can use the formula for permutations. Since repetition is allowed, we use the formula for permutations with repetition, which is n^r, where n is the total number of options (10 in this case) and r is the number of digits in each combination (3 in this case). Therefore, the total number of 3-digit combinations that can be made from the numbers 1-9 is 10^3 = 1000.
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6
Only one.
10
Six combinations: 123, 132, 213, 231, 312, 321
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.