Twenty-four. You have four choices for the first digit. Once selected, you have three choices for the second digit. Then two for the third and then the last digit.
Multiply 4 X 3 X 2 X 1 and you get 24, which is 4! or "4 factorial".
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The answer is 10P3 = 10!/(10-3)! = 10*9*8 = 720
This is permutations with repetition. The answer is 4^4 = 256 total permutations. Since 2 of the digits used are odd (and 2 are even), then half of the possibilities will be odd: 128 odd numbers.
The answer is 26*26*26*10*10*10 = 17,576,000
If you wish to use a brute-force method, assuming the digits go from 0 to 9 there are 10,000 possible permutations.
If it alternates and you start with a letter, then there are 11,232,000 permutations. Then if you start with a number and alternate, there are another 11,232,000 permutations, for a total of 22,464,000 permutations. If you exclude the letters I, S, B, and O (because they look kind-of like 1, 5, 8, & 0 - kind-of important on license plates) then you are down to 6,652,800 & 13,305,600 respectively.