The order of the digits in a combination does not matter. So 123 is the same as 132 or 312 etc.
There are 10 combinations using just one of the digits (3 times).
There are 90 combinations using 2 digits (1 once and 1 twice).
There are 120 combinations using three different digit.
220 in all.
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There are 9C3 = 84 combinations.
There are 9C3 = 10*9*8/(3*2*1) = 120 of them.
To calculate the number of 4-digit combinations using the digits 1, 3, 5, and 7 exactly once each, we can use the permutation formula. There are 4 choices for the first digit, 3 choices for the second digit, 2 choices for the third digit, and 1 choice for the fourth digit. Therefore, the total number of combinations is 4 x 3 x 2 x 1 = 24. So, there are 24 possible 4-digit combinations using the digits 1, 3, 5, and 7 exactly once each.
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.
45*44*43*42*41/(5*4*3*2*1) = 1,221,759