.
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.
Solved.
---------------
On Permutations, see these introductory lessons
- Introduction to Permutations
- PROOF of the formula on the number of Permutations
- Simple and simplest problems on permutations
- Special type permutations problems
- Problems on Permutations with restrictions
in this site.
Also, you have this free of charge online textbook in ALGEBRA-II in this site
- ALGEBRA-II - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic "Combinatorics: Combinations and permutations".
Save the link to this textbook together with its description
Free of charge online textbook in ALGEBRA-II
https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson
into your archive and use when it is needed.