Since 0 cannot be in the first place of the 4-digits number, we have 9 x 10 x 10 x 10 = 9,000 combinations.
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Assuming the digits cannot be repeated, there are 7 combinations with 1 digit, 21 combinations with 2 digits, 35 combinations with 3 digits, 35 combinations with 4 digits, 21 combinations with 5 digits, 7 combinations with 6 digits and 1 combinations with 7 digits. That makes a total of 2^7 - 1 = 127: too many for me to list. If digits can be repeated, there are infinitely many combinations.
Oh, dude, you're hitting me with some math vibes here. So, if you have 6 digits to choose from to make a 4-digit combination, you can calculate that by using the formula for permutations: 6P4, which equals 360. So, like, you can make 360 different 4-digit combinations from those 6 digits. Math is wild, man.
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If you exclude numbers starting with zero then the first digit must be between 1 and 9 (i.e. 9 combinations). The remaining 9 digits can be any value between 0 and 9 (i.e. 10 combinations).So you can have 9x109 = 9,000,000,000 combinations.
If you have 4 positions, each of which can hold any of the ten digits, you have 10 to the power 4 combinations. If you can have only 4 different digits, you have 4 to the power 4 different combinations.