anon11: How do you know when to use combination and when to use permeation?
You use permutations when order matters.
If you need to randomly choose 4 books from 10 because you are going to pack them and send them as a gift. Then the order that they are chosen doesn't matter.
The number of combinations would be 10C4 = 210 (from my calc)
If, on the other hand, you want to choose 4 books from 10 because you are going to read them sequentially. Then the order does matter.
B1, B7, B2, B5 is different from B5, B7, B1, B2 because they will be read in a different order.
So the number of permutations will be 10P4 = 5040
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Now I am going to discuss it a little further.
There are 210 ways to choose 4 books from 10
There are 4P4 (or 4!) ways of ordering those 4 book = 24
210*24 = 5040
+118723 
