SOLUTION: Find the total number of different 4-digit numbers using all the digits in the number 4129. Multiple choice answers: a. 24 b. 12 c. 18 d. 6

It is the number of all permutations of four digits, which is 4*3*2*1 = 24.        ANSWER  


Another way to present it is to say that


    - any one of four digits can occupy the 1-st position;

    - any one of the remaining three digits can occupy the 2-nd position;

    - any one of the remaining two digits can occupy the 3-rd position;

    - and there is only one way to fill the last, 4-th position,


which gives you  4*3*2*1 options, in all.