.
In 400*399*398 ways.
This time the positions are DISTINGUISHABLE,
so the order DOES MATTER.
Therefore, we use PERMUTATIONS, and do not use combinations.
Why the formula goes in this way, I just EXPLAINED several times in my previous posts.
Happy learning (!)
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See the lesson
- Special type permutations problems
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 lesson is 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.