Using the word "combinations" in the English sense (as opposed to mathematical sense the expert has used) where it often used in the mathematical sense of the word "permutations":
Assuming the hundreds digit must be at least 1 (eg 99 = 099 is not considered a three digit number), then:
9 x 10 x 10 = 900.
If the digits can repeat, then there are 256 possible combinations. If they can't repeat, then there are 24 possibilities.
Number of 7 digit combinations out of the 10 one-digit numbers = 120.
There are 210 4 digit combinations and 5040 different 4 digit codes.
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.
To calculate the number of different 4-digit combinations that can be made using numbers 0 through 9, we use the concept of permutations. Since repetition is allowed, we use the formula for permutations with repetition, which is n^r, where n is the number of options for each digit (10 in this case) and r is the number of digits (4 in this case). Therefore, the number of different 4-digit combinations that can be made using numbers 0 through 9 is 10^4, which equals 10,000 combinations.
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
If the digits can repeat, then there are 256 possible combinations. If they can't repeat, then there are 24 possibilities.
If you can repeat the numbers within the combination there are 10,000 different combinations. If you cannot repeat the numbers within the combination, there are 5040 different combinations.
Number of 7 digit combinations out of the 10 one-digit numbers = 120.
There are 210 4 digit combinations and 5040 different 4 digit codes.
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.
9
1
if its not alphanumeric, 999999 variations
15
10,000
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.